Astronomy & Astrophysics | Physical Research Laboratory

2026

Stellar Feedback as Regulator of Star Formation in Galactic Clouds

Date: 2026-02-12
Speaker: Dr. Ram Kesh Yadav
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Recent surveys and simulations show that stellar feedback operating through radiation, winds, H II region expansion, outflows, and supernovae regulates where, when, and how efficiently molecular gas forms stars in the Galaxy. In this talk, I outline the current physical picture of feedback-driven cloud evolution, emphasizing shell and filament compression, photoevaporation and dispersal, turbulence injection, and the feedback-gravity competition that can both trigger secondary star formation and suppress further collapse. I use observational diagnostics that connect feedback to dense-gas formation, including dense-gas tracers, kinematics, and dust and infrared constraints, to highlight recent results. These include FIRESTORM I, the first paper of the FIRESTORM project, which targets a feedback-shaped environment to quantify how feedback restructures dense gas and redistributes star formation activity. Evidence from simulations and cloud-lifecycle measurements suggests that the net impact of feedback depends on geometry and evolutionary timescale, motivating multi-tracer mapping and kinematically resolved tests that link cloud structure to star formation.

FUTURE PLANS FOR GLOBAL TRANSIENT NETWORKS

Date: 2026-02-06
Speaker: Prof. David A H Buckley
Venue: Seminar Room No. 113/114 (Thaltej Campus)

Abstract

<div>I will review plans for global transient and detection networks of the future. The BRICS+ astronomy flagship programme, entitled the BRICS Intelligent Telescope and Data Network (BITDN), aims to harness existing and future facilities within BRICS+ countries for automated transient observations, both their detection and followup. Likewise a smaller Africa initiative, the African Integrated Observation Network (AIOS) has similar aims, utilizing continental facilities in northern, eastern and southern Africa. The major next development in transient and variable detections will inevitably push to higher cadences and better sky coverage, as envisaged with GOTTA: a Global Open Transient Telescope Array, a new Chinese-led project.  The current concept consists of 135 wide field 1-m modified Schmidt telescopes, each with a 25 sq degree field of view, with effective 18k x 18k x 15 micron CMOS cameras. Each camera has one dedicated filter (e.g. g,r or i). Ideally, these telescopes will be situated in groups of 3 in both hemispheres and with sufficient longitude range to achieve all-sky coverage, with a cadence of less than an hour. Larger aperture (2-4-m class) telescope will be used for spectroscopic followup, while some smaller aperture and field of view telescopes can support photometric followup.</div>

Probing thermonuclear X-ray bursts and burst-disk interaction in accreting neutron stars

Date: 2026-02-05
Speaker: Dr. Manoj Mandal
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Neutron stars in low-mass X-ray binaries provide unique laboratories for studying matter under extreme gravity, density, and magnetic fields. These systems consist of a neutron star accreting matter from a low-mass companion star, typically through Roche-lobe overflow. In such systems, the neutron star usually possesses a relatively weak magnetic field (~10⁷-10⁹ G), allowing the accreted material to spread over the stellar surface rather than being funneled directly onto the magnetic poles. As a result, the accumulated fuel can undergo unstable nuclear burning, leading to sudden thermonuclear explosions on the neutron star surface, observed as thermonuclear X-ray bursts. In some energetic bursts, the radiation is strong enough to temporarily lift the photosphere, causing a photospheric radius expansion (PRE). Some bursts also show burst oscillations, rapid periodic variations caused by localized hotspots in the burning layer. In this talk, I will present studies of thermonuclear X-ray bursts, including photospheric radius expansion events and evidence of burst-disk interaction. I will also discuss the results from a newly discovered accreting millisecond X-ray pulsar with numerous bursts, where spectral and timing analyses reveal disk reflection and the first detection of burst oscillations. Overall, these studies demonstrate how thermonuclear bursts can be used as powerful tools to probe neutron star properties and accretion physics in extreme environments.

X-ray optics development, testing, and calibration for current and future missions at PANTER

Date: 2026-02-03
Speaker: Dr. Vadim Burwitz
Venue: Seminar Room No. 113/114 (Thaltej Campus)

Abstract

MPE together with its PANTER X-ray test facility is involved in the development, testing and calibration of X-ray optics, Detectors, complete telescopes for most existing  X-ray observatories and future large missions. I will present the X-ray test facility. I will also describe the missions and technologies they use as well as the types of measurements that are performed to ensure the flight readiness of the missions as well as providing as sturdy on ground calibration  to support the in-flight calibrations. These activities now also are coordinated the IACHEC  cross mission calibration group.

Discovery of Changing-look Behavior in AGN NGC3822: A Long-term Multiwavelength Study

Date: 2026-01-22
Speaker: Narendranath Layek
Venue: Room no. 113/114, Thaltej Campus

Abstract

Active galactic nuclei are the most luminous and energetic sources in the universe, powered by the accretion of matter onto the supermassive black holes (SMBHs) located at the centers of the host galaxies.  In the optical/UV range, the AGNs are commonly classified as type 1 or type 2 based on the widths of their optical emission lines. Type 1 AGNs show both broad emission lines (BELs) and narrow emission lines (NELs), whereas type 2 AGNs show only NELs in their UV/optical spectra. In recent years, several tens of subclasses of AGNs have been discovered, exhibiting dramatic optical and X-ray spectral variability on timescales ranging from months to decades. These are known as changing-look AGNs and are currently an open issue in AGN physics. In this seminar, I will present a 17-year (2008–2025) multiwavelength study of the changing-look AGN NGC 3822, combining X-ray and UV data, along with optical observation from the Very Large Telescope and the Himalayan Chandra Telescope. Long-term optical monitoring reveals clear evolution in the emission-line properties, including the appearance and disappearance of broad Balmer lines, confirming the changing-look nature of the source. I will discuss the observed spectral-state transitions, their connection to X-ray/UV variability, and what these results imply about the possible drivers of changing-look behaviour, such as variable obscuration and changes in the accretion rate.

2025

Understanding Dwarf Novae Type Cataclysmic Variables

Date: 2025-12-31
Speaker: Ayush Rana
Venue: 113/114, Thaltej Campus

Abstract

Cataclysmic Variables (CVs) are close interacting binary systems consisting of a white dwarf accreting material from a late-type main-sequence companion via Roche-lobe overflow. Due to their small orbital separations, typically a few solar radii, CVs exhibit short orbital periods of a few hours and display strong photometric variability. Dwarf novae are a prominent subclass of CVs and are characterised by recurrent optical outbursts with amplitudes of 2–6 magnitudes and recurrence timescales ranging from weeks to months. These outbursts are understood with the disk instability model (DIM), in which thermal–viscous instabilities in the accretion disk drive transitions between quiescent and outburst states. Dwarf novae provide an excellent laboratory for studying accretion disk physics and time-dependent disk evolution. In eclipsing systems, changes in the disk structure can be probed using eclipse depth and out-of-eclipse flux measurements. Recent high-cadence photometric studies have revealed hysteresis behaviour in eclipse fraction diagrams, indicating a temporal lag between variations in mass accretion rate and disk radius during outburst cycles. In this talk, I present an overview of cataclysmic variables with emphasis on dwarf novae, their outburst mechanisms, and observational diagnostics of accretion disk evolution. I also outline the motivation and objectives of ongoing work focused on studying the evolution of accretion disks in eclipsing dwarf novae.

Understanding the Spectral Energy Distribution (SED) of Blazars

Date: 2025-12-29
Speaker: Ashad Ahmad
Venue: 113/114, Thaltej Campus

Abstract

Relativistic jets in blazars are sites of extreme plasma conditions, efficient non-thermal particle acceleration, and broadband radiation extending from radio to very-high-energy γ-rays. Their spectral energy distributions (SEDs) exhibit a characteristic double-humped structure, commonly interpreted as synchrotron emission at low energies and inverse-Compton or hadronic processes at high energies. The observed luminosities, rapid variability, superluminal motion, and one-sidedness of the jet impose strong constraints on the size, speed, and geometry of the emitting region, requiring relativistic beaming and compact dissipation zones within the jet. Leptonic and hadronic emission models are commonly employed to reproduce blazar SEDs and multi-wavelength variability, but these models typically rely on simplified assumptions for the underlying non-thermal particle energy distributions. Such non-thermal spectra cannot arise from purely thermal processes, indicating the presence of efficient particle acceleration operating in relativistic jet environments.

Stellar Streams in the Milky Way's Halo

Date: 2025-12-26
Speaker: Arvind
Venue: 113/114, Thaltej Campus

Abstract

The Milky Way galaxy is our home galaxy, hosting our solar system and billions of other stars. One of the key questions in galactic astronomy is to understand the formation and evolution of galaxies. As we are part of the Milky Way, it provides us with a unique opportunity to study its components in great detail, allowing us to gain a deeper understanding of how galaxies form and evolve in general. According to the cosmological paradigm, galaxies form through hierarchical accretion of lower-mass galaxies. The recently discovered stellar streams in the Milky Way's halo are direct evidence of these merger events. Stellar streams form due to the tidal disruption of a globular cluster or dwarf galaxy that orbits the Milky Way. In this talk, I will discuss about the stellar streams, how they are detected, and what information we obtain from their spectroscopic follow-up. I will also discuss about the target selection for the spectroscopic observation of the stream stars. Determining the age of the stellar streams is crucial because they serve as a record of the Milky Way's growth, revealing its merger history and assembly timeline.

Spectro-Polarimetry of Symbiotic Star

Date: 2025-12-24
Speaker: Sube Singh Gurjar
Venue: 113/114, Thaltej Campus

Abstract

Symbiotic stars are interacting binary systems composed of a cool red giant and a hot white dwarf. Material is transferred from the giant to the dwarf through a stellar wind or in some systems,via Roche-lobe overflow. An intriguing fact of symbiotic systems is their extremely low known population; only around 300 such system are known against their predicted population of around 300,000 in our Galaxy. Moreover,these exhibit unique emission features at 6825 Å and 7082 Å, which are caused by the Raman-scattering of O VI doublet at 1032 and 1038 Å. As these features are a result of scattering, they show polarization signatures and hence can be used to probe the morphology and kinematics of the systems. We plan to undertake a long term spectro-polarimetic observational study of such systems, wherein any temporal variability of these Raman scattered features would be explored using spectro-polarimetric observations from ProtoPol instrument on PRL 2.5m telescope. In addition, we are upgrading the existing MFOSC-P instrument on PRL 1.2m telescope with newer spectro-polarimetric capabilities.

Revealing the State of the Early Intergalactic Medium through 21-cm Signal Analysis

Date: 2025-12-22
Speaker: Dr. Raghunath Ghar
Venue: 113/114, Thaltej Campus

Abstract

Measurements of the power spectrum of the redshifted 21-cm signal from the Epoch of Reionization (EoR) provide crucial insights into the ionization and thermal states of the intergalactic medium (IGM) and depend sensitively on the nature of the early radiation sources. Recent upper-limit constraints from SKA precursor experiments such as MWA, HERA, and LOFAR have already begun to exclude several extreme reionization models (e.g., Ghara et al. 2020, 2021). In these models, the 21-cm signal fluctuations are primarily driven by rare, large ionized or emission regions in the early Universe. With forthcoming, more stringent upper limits, particularly from small-scale power spectrum measurements by the SKA, the constraints on the IGM state during the EoR are expected to improve significantly. In this presentation, I will discuss which reionization scenarios and IGM conditions are likely to be ruled out first with SKA observations. I will also summarize the current understanding of the IGM properties during the EoR based on results from ongoing 21-cm signal experiments.

Multiple Emission-Line Diagnostics of the Accretion Process in Young Stars

Date: 2025-12-04
Speaker: Kushagra Srivastav
Venue: 113/114, Thaltej Campus

Abstract

Young stars consist of a prestellar core, a circumstellar disk, and an envelope. Material from the envelope accretes to the disk, and through angular-momentum loss or removal, disk material is transported onto the central star. Circumstellar disks are therefore central to both stellar growth and planet formation, making them crucial for understanding the early stages of stellar evolution. In low-mass pre-main-sequence stars, accretion proceeds through magnetospheric funnels that channel disk material onto the stellar surface, generating accretion shocks that result in an excess emission in the UV region and strong emission lines. Classical T Tauri stars (CTTSs) show highly variable accretion across multiple timescales. The origins of this variability remain unknown. In this seminar, I will present a detailed analysis of a classical T Tauri star using multiple emission-line diagnostics to investigate its accretion properties and discuss various line diagnostics related to star-disk interaction processes.

Propagation of Solar Energetic Particles in the Heliosphere

Date: 2025-11-27
Speaker: Ritik Dalakoti
Venue: 113/114, Thaltej Campus

Abstract

Solar Energetic Particles (SEPs) are charged particles with energies ranging from keV to several GeV, accelerated during solar flares and CMEs. Understanding SEPs is crucial for space weather. These particles generally follow the large-scale interplanetary magnetic field, however, heliospheric magnetic turbulence repeatedly scatters them. This scattering governs SEP propagation, and diffusion parameters such as the parallel diffusion coefficient quantify the strength of diffusion along magnetic field lines. By combining X-ray spectral observations from the Solar X-ray Monitor (XSM) onboard Chandrayaan-2 with particle flux measurements from multiple spacecraft, and by modelling the particle propagation event, we experimentally determine the SEP diffusion coefficient.

Random encounters in asteroseismology and their impact on a scientific career

Date: 2025-11-18
Speaker: Dr. Peter De Cat
Venue: 113/114, Thaltej Campus

Abstract

As a little boy, I was already fascinated by the magnificent light show of the twinkling stars in the night sky. My scientific career started almost 30 years ago when, as a physics student at KU Leuven (Belgium), I prepared my master's thesis under the supervision of Conny Aerts. Thanks to her enthusiasm, I started to look into asteroseismology, where the stars reveal the secrets of their hidden interior through the way they vibrate. In this presentation I would like to demonstrate how my interest in stellar pulsations in combination with a few random encounters have led to the two projects that I consider my greatest scientific achievements to date, namely the close collaboration with colleagues from both China (via the LAMOST-Kepler project) and India (via the BINA project). The strength of these projects will be demonstrated by some scientific results in different domains.

Long term X-ray Spectral study of Seyfert Galaxy Mrk 530

Date: 2025-11-17
Speaker: Priyadarshee P. Dash
Venue: 113/114, Thaltej Campus

Abstract

Active Galactic Nuclei (AGN) are among the most luminous astronomical sources, powered by accretion of matter onto a Supermassive Black Hole (SMBH). Their X-ray emission arises from the upscattering of optical/UV photons from the accretion disk by a hot corona of relativistic electrons, producing a power-law spectrum, which is modified by absorption and reflection features. In some AGN, an excess of photons above the standard power-law can be observed below the 2 keV energy range, termed as Soft Excess. The origin of this Soft excess is still a matter of debate, and various theories have been proposed to explain this excess emission. The warm corona model attributes this emission to arise from the inverse Comptonizaton of the disk photons in an optically thick, warm (0.1 - 1 keV) corona. Alternatively, the blurred ionized reflection scenario explains the soft excess to be a result of the relativistic blurring of the reprocessed emission from the disk due to the effects of strong gravity near the SMBH. More recently, hybrid models combining both warm Comptonization and ionized reflection components have also been proposed.

Broad-band temporal and spectral study of Blazars

Date: 2025-11-06
Speaker: Avik Kumar Das
Venue: 113/114, Thaltej Campus

Abstract

Blazars are the most powerful subclass of active galactic nuclei in which observed emissions are highly Doppler boosted and observable across the entire accessible electromagnetic spectrum ranging from radio to gamma-rays, with diverse variability timescales spanning across minutes to years. The AGN activity level appears to vary on various time-scales. To date, most of the studies have focused mainly on high activity episodes, thereby offering a limited view of the source physical conditions. In our recent work we studied long-term broadband temporal and spectral study of a TeV BL Lac object TXS 0518+211 using 16 years of simultaneous optical, UV, and X-ray observations. In this blazar we find the complex nature of jet-dominated emission processes and suggest for more than one emission zone contributing to the observed broadband spectral energy distribution (SED). In this talk, I shall also present the results based on the photometric and spectroscopic observations acquired from Mt. Abu telescopes.

On-sky characterization and performance verification of ProtoPol

Date: 2025-10-23
Speaker: Arijit Maiti
Venue: 113/114, Thaltej Campus

Abstract

ProtoPol is a medium-resolution echelle spectro-polarimeter developed for the PRL 1.2m and 2.5m telescopes at Mt Abu observatory, India. ProtoPol provides a spectral resolution in the range of âˆ¼0.4 - 0.75ËšA across various orders in the visible wavelength range of 4000-9600ËšA. On PRL 2.5m PRL telescope, an SNR of 10 is achieved for Vmag âˆ¼ 13.5 source in 1 hour of integration time, and its throughput is estimated to be âˆ¼6% including all the contributing factors such as atmospheric transmission, telescope reflectivity, instrumentâ€™s optics, CCD efficiency, etc. ProtoPol achieved a linear polarization accuracy â‰ˆ 0.1 âˆ’ 0.2% in 2 hours of integration time for a source with Vmag â‰ˆ 8. The instrumental polarization is determined to be around 0.1%. Several Science observation campaigns were conducted with ProtoPol to demonstrate the capabilities of the instrument. A sample of Herbig Ae/Be stars, classical Be stars, Symbiotic stars, and AGB/post-AGB stars were observed over the period of one and a half years for their spectro-polarimetry measurements covering various physical mechanisms such as line polarization in Herbig and classical Be stars, polarization in Raman scattered features in Symbiotic stars, as well as continuum polarization in AGB/post-AGB stars to verify the polarization performance of the instrument.

Exploring GRB Localisation Capability of Proposed Daksha Mission using Coded Mask Imaging with its Low-Energy Detectors and Their Development.

Date: 2025-10-16
Speaker: Ashish Kumar Mandal
Venue: 113/114, Thaltej Campus

Abstract

Gamma Ray Bursts (GRB) are short, intense extragalactic gamma-ray flashes. GRBs are of two kinds-long GRBs (burst duration >2 sec) thought to be produced due to core collapse of rapidly rotating massive stars, short GRBs (duration <2 sec) believed to be produced due to merger of binary compact objects like neutron star-neutron star (NS-NS) or black hole-neutron star (BH-NS). Daksha is India’s proposed high-energy transient mission aimed at detecting and characterizing electromagnetic counterparts (EMCs) of gravitational wave (GW) events and GRBs. In its initial configuration, Daksha can localize short GRBs to within 5–10 degrees using a projection method. Daksha’s scientific potential could be greatly enhanced if onboard GRB localization within 1 degree is made possible, which can be potentially achievable using the Coded Aperture Mask (CAM).

Multi-wavelength Variability and Quasi Periodic Oscillations (QPOs) in Blazars

Date: 2025-10-13
Speaker: Prof. Alok Gupta
Venue: 113/114, Thaltej Campus

Abstract

This is the age of multi-wavelength (MW) time domain astronomy in which the transient astronomical sources are of great interest due to their rapid change in flux, spectra and polarization. Simultaneous MW observation of a particular transient source over an extended period of time has importance to understand the emission mechanism in different electromagnetic (EM) bands. Blazars are among one of the most favourite astronomical transient objects, because they emit radiation in the complete EM spectrum, and their flux, spectra and polarization are highly variable.

NICER insight into the High-Energy Universe

Date: 2025-08-07
Speaker: Dr. Gaurava Kumar Jaiswal
Venue: 113/114, Thaltej Campus

Abstract

Since its deployment to the International Space Station in June 2017, the Neutron Star Interior Composition Explorer (NICER) has significantly advanced our understanding of compact objects in the X-ray sky. With exceptional timing and spectral capabilities in the 0.2 - 12 keV range, NICER enables detailed studies of accretion-powered and magnetically driven phenomena. A major focus has been on thermonuclear X-ray bursts -brief, intense explosions caused by unstable burning of hydrogen and helium on neutron star surfaces, which offer key insights into the physics of dense matter. This talk will highlight crucial findings from eight years of NICER observations, covering burst behavior, accretion dynamics, X-ray transients, and magnetar activities. It will also discuss efforts to understand the interaction between compact objects and their optical companions through X-ray and optical observations. Finally, I will present recent updates on the long-term calibration of the JEM-X instruments aboard ESA's INTEGRAL mission and ongoing progress toward establishing the INTEGRAL Legacy Archive.

An overview of compact X-ray binaries

Date: 2025-08-06
Speaker: Dr. Gaurava Kumar Jaiswal
Venue: 113/114, Thaltej Campus

Abstract

Compact objects represent some of the most extreme environments in the universe. When these dense remnants are part of a binary system, they can draw in matter from their companion stars through a process known as accretion. This transfer of material releases vast amounts of energy, making these systems strong sources of X-rays. Compact X-ray binaries, in which a neutron star or black hole accretes matter from a companion, offer unique opportunities to study energetic processes like accretion dynamics, outbursts, and thermonuclear bursts on neutron star surfaces. This talk will introduce the fundamental concepts behind accretion, compact binaries, and related phenomena, and will highlight why these systems are key to advancing our knowledge of high-energy astrophysics.

Advancing the Warm Calibration Unit for METIS on ELT: From Design Finalization to MAIT Phase

Date: 2025-08-04
Speaker: Dr. Vipin Kumar
Venue: 113/114, Thaltej Campus

Abstract

METIS, an advanced mid-infrared imager and spectrograph for the wavelength range 2.9-13.5 microns (astronomical L, M, and N-band), stands as one of the three science instruments at the Extremely Large Telescope (ELT). It will provide diffraction-limited imaging, coronagraphy, high-resolution integral field spectroscopy, and low and medium-resolution slit spectroscopy. Within the collaborative international METIS consortium, the University of Cologne is responsible for the design, manufacturing, and integration of the Warm Calibration Unit (WCU) of the instrument. The main role of WCU is to provide a stable and controllable reference signal to METIS that will allow troubleshooting and calibrating the response of METIS in various observing modes. In this talk, I will present the key requirements from METIS that drive the design of the WCU, followed by an overview of its optical and opto-mechanical design, and functional role within the instrument. The final part of the presentation will focus on the current status of the Manufacturing, Assembly, Integration, and Testing (MAIT) phase, including a brief discussion on the alignment verification plans for the offner relay optics of WCU.

Broad-band study of the SMC pulsar RX J0032.9-7348 during its X-ray brightening in 2024

Date: 2025-07-31
Speaker: Dr. Birendra Chhotaray
Venue: 113/114, Thaltej Campus

Abstract

Accretion-powered X-ray pulsars (XRPs) are magnetized neutron stars that are part of X-ray binary (XRB) systems. The XRPs emit X-rays by accreting mass from their binary companion. These pulsars are characterized by intense magnetic fields, typically ranging from ~ 10^12-10^14 G, which direct the infalling material toward their magnetic poles, where most X-ray photons are generated. When the magnetic and spin axes are misaligned, the resulting emission is observed as pulsations from the neutron star. RX J0032.9-7348, an X-ray transient, was first detected by ROSAT in 1993; however, its properties are largely unknown. After years of inactivity, the source entered an X-ray bright phase in 2024. We observed it during this phase using NuSTAR and NICER. In this talk, I will present the timing and spectral properties of RX J0032.9-7348 during its 2024 outburst and discuss our ongoing and future work.

Census and Characterization of Hot and Variable Stars in Star Clusters

Date: 2025-07-28
Speaker: Dr. Arvind K. Dattatrey
Venue: Online

Abstract

Hot and variable stellar populations in star clusters such as blue straggler stars (BSSs), blue lurkers, horizontal branch (HB) stars, and white dwarfs (WDs) offer critical insights into stellar evolution, binary interaction mechanisms and the dynamical evolution of stellar systems. In this talk, I will shed light on the characteristics and formation pathways of these non-canonical stars with a few case studies. I will discuss the detection of extremely low-mass white dwarfs (ELM WDs) as companions in BSS binary systems in Globular cluster NGC 362. In NGC 362, the radial distribution of BSSs exhibits strong central concentration, classifying the cluster as dynamically evolved and likely in a post-core-collapse phase. We also report the first identification of blue lurkers in a globular cluster. Two-component SED modeling reveals their companions to be low-mass and ELM WDs with short cooling ages (< 4 Myr), suggesting a recent formation event likely triggered by dynamical interactions during core collapse. In addition, we identify high-mass WDs in NGC 362, which may have originated via white dwarf–white dwarf mergers, a rare but theoretically predicted evolutionary channel in dense stellar environments. In the open cluster NGC 2420, four binary systems were identified, including two BSS binaries, one BSS–ELM WD system, and one HB–ELM WD system. These systems are consistent with formation via Case A/B mass-transfer pathways, indicative of stable binary evolution in low-density environments. Ongoing time-series photometric analysis has led to the identification of multiple variable stars, including pulsating and eclipsing systems, particularly among the BSS and HB populations. These variables provide independent diagnostics of internal stellar structure, angular momentum evolution, and rotational modulation. I will also discuss the ongoing spectroscopic analysis of BSS–RGB binary systems to further constrain the mass-transfer origin of BSSs, for understanding the role of binary evolution in shaping the hot stellar populations observed in star clusters.

Characterizing AGN and dual AGN duty cycles in SMUGGLE isolated merger simulations

Date: 2025-07-17
Speaker: Mr. Jay Motka
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Observational data show strong correlations between the mass of supermassive black holes and the properties of their host galaxies, leading to the idea of SMBH-galaxy co-evolution. Galaxy mergers are interesting events as gravitational torques during mergers drive gas towards galactic nuclei, which can enhance AGN activities. This leads one to question: to what extent do mergers enhance AGN activities compared to isolated galaxies, and how does this impact the galactic environment? In this talk, I shall elaborate on our efforts to answer this question by introducing Stars and MUltiphase Gas GaLaxiEs (SMUGGLE), a framework based on the hydrodynamic code AREPO with a multiphase interstellar medium. I shall talk about the limitations of traditional simulations, improvements in SMUGGLE, and other simulation specifications. Our efforts in characterizing the duty cycles of AGNs and dual AGNs during mergers have shown that Multiphase ISM in SMUGGLE yields highly variable accretion rates with short duty cycles. In mergers, the mean active phase timescales are in the order of 0.1 Myr to 1 Myr, and dual AGNs seem to be active for 10-20 per cent of the simulation time before the black holes merge. Differences in black hole masses, galaxy morphologies, and wind speeds do show a significant impact on AGN and dual AGN activities. I shall conclude with a summary of current results, as well as ongoing and future work.

Observing Solar Flares with SUIT: A New Window into the Near-Ultraviolet Sun

Date: 2025-06-19
Speaker: Soumya Roy
Venue: Online

Abstract

Solar flares are among the most energetic phenomena in the solar system, driving rapid restructuring of magnetic fields and heating across the solar atmosphere. While much progress has been made in understanding flares through observations in X-rays and extreme ultraviolet (EUV), the near-ultraviolet (NUV) range remains relatively unexplored—despite its sensitivity to key layers like the lower chromosphere and upper photosphere, where much of the flare energy is deposited. The Solar Ultraviolet Imaging Telescope (SUIT), onboard India’s Aditya-L1 mission, is the first instrument to provide full-disk solar imaging in multiple NUV passbands, opening a new window into this critical regime. In this talk, I will first outline the current state of flare research and highlight the observational and diagnostic gaps that exist in the NUV. I will then introduce SUIT’s science objectives in the context of these challenges. As part of my PhD, I contributed to the initial design of SUIT. I will also present our first results from SUIT, and discuss how these data can inform models of chromospheric heating and flare energy transport. This work sets the stage for SUIT’s long-term contribution to high-cadence, multi-height solar flare studies.

Multi-mode study of exotic stellar populations in dense stellar systems

Date: 2025-06-12
Speaker: Gaurav Singh
Venue: Online

Abstract

I will present a systematic study of exotic stellar populations-Blue Stragglers (BSS) and Extreme Horizontal Branch (EHB) stars-in dense environments of the Milky Way and satellite dwarf galaxies. Despite decades of research, their formation channels, binary companions, and chemical evolution remains poorly understood due to their diverse properties across different environments. These UV-bright objects have now become accessible through new facilities like AstroSat/UVIT and public catalogs, revealing previously unknown extreme systems that challenge existing stellar models. To resolve these puzzles, I propose a coordinated multi-wavelength campaign utilizing high and medium-resolution spectroscopy (ground-based optical/infrared observatories), UV diagnostics (HST, UVIT/AstroSat, Swift/UVOT), and astrometry (Gaia and HST). This approach will uncover binary interactions, atmospheric anomalies, and evolutionary pathways-providing critical insights into stellar rejuvenation mechanisms and revealing new characteristics of these stellar populations.

The Secret Lives of Galaxies: From Dusty Starbursts to Buried Black Holes

Date: 2025-05-20
Speaker: Dipanjan Mitra
Venue: Online:https://imeet.vconsol.com/join/9277507210?be_auth=NDc5OTIx

Abstract

Abstract - Galaxies are the building blocks of the Universe, and some of the most massive and mysterious ones formed when the Universe was just a few billion years old. Many of these early galaxies were rich in dust and forming stars at high rates, yet hidden from view in visible light. These dusty star-forming galaxies (DSFGs) played a key role in building today’s giant elliptical galaxies, and understanding them is essential to piecing together the story of galaxy formation. In my recent work, I used data from space and ground-based telescopes (JWST, Euclid, Herschel, and LSST) to show how we can trace the stars, dust, and AGN activity in these galaxies across cosmic time, and developed physical models. In this talk, I shall demonstrated that by combining multi-wavelength data (from ultraviolet to far-infrared) we can get a complete picture of how galaxies evolve, even when they are deeply buried in dust. I shall also discuss the role deep radio surveys in unveiling obscured active galactic nuclei (AGN) at high redshifts.

Atmospheric evaporation from exoplanets

Date: 2025-05-07
Speaker: Dr. Gopal Hazra
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The habitability of any planet is decided by a complex evolution of its interior and atmosphere. Recently in many observations, it has been found that close-in exoplanets are going through significant atmospheric evaporation, which could affect the overall evolution of the exoplanet atmosphere. This atmospheric evaporation from exoplanets is very much dependent on the plasma and radiation environment of their parent stars (e.g., stellar radiation, stellar wind, stellar flares, and Coronal Mass Ejections (CMEs). In this talk, I will explain different physical processes (e.g., Jeans escape, hydrodynamic escape) by which an exoplanet can lose its atmosphere. One major process that leads to significant loss of exoplanetary atmosphere is the stellar radiation-driven atmospheric outflow. Once planetary outflow is initiated from the planet by stellar radiation, it further interacts with the stellar wind shaping up the exoplanetary atmosphere (sometimes producing a comet-like structure) and its atmospheric mass-loss rate. Moreover, flares and coronal mass ejections (CMEs) from the star will also have a great impact on planetary evaporation and mass loss. I will present our newly developed 3D radiation magnetohydrodynamics model where we have implemented a self-consistent radiative transfer of incident stellar radiation to simulate planetary outflow and its interaction with the stellar wind, CMEs, and flares. I will show that radiation-driven planetary outflow alone can not explain the observed transit signatures & corresponding mass-loss rate, but the interaction with the stellar wind/coronal mass ejections can explain the observed mass-loss rate and transit for many exoplanets. I will also discuss briefly the effect of stellar and planetary magnetic fields on the atmospheric mass-loss rate and corresponding observational signatures.

Time-Dependent Modeling of Extreme Gamma-Ray Flares of Blazars

Date: 2025-05-01
Speaker: Anton Dmitriev
Venue: Online

Abstract

Blazar flares offer a unique window into the extreme physics of relativistic outflows, including particle acceleration and the origin of multi-wavelength (MWL) emission. A key approach to studying these processes is physical modeling of varying blazar jet emission. Many numerical codes employ a kinetic framework to track particle spectrum evolution under various physical effects. We have developed EMBLEM (Evolutionary Modeling of BLob EMission), a versatile radiative code based on time-dependent particle (re-)acceleration, escape, radiative cooling, and adiabatic expansion. This code allows us to self-consistently connect low state and flaring emission. Based on a leptonic framework, the code incorporates synchrotron self-Compton (SSC) and external Compton (EC) scenarios. We showcase its application to (1) modeling extreme gamma-ray flares in blazars such as Mrk 421 and 3C 279, and (2) searching for internal gamma-ray opacity signatures in high-redshift blazars.

Magnetic Accretion Signatures in High-Field Cataclysmic Variables

Date: 2025-04-29
Speaker: Akash Sundriyal
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Cataclysmic Variables (CVs) are examples of semi-detached binary star systems characterized by the flow of stellar material from the companion star to the primary (white dwarf). If the primary possesses a significant magnetic field, the accretion dynamics can be drastically altered. In particular, polar magnetic CVs, characterized by their strong magnetic fields are often marked by the lack of an accretion disk and also exhibit unique observational signatures, such as highly polarized radiation, synchronous rotation, etc. In this talk, I will discuss the properties of these high field systems and the interaction between the magnetic field and the accretion flow which can lead to complex phenomena, including emission of X-rays and cyclotron radiation. I will then explain some methods used to probe the magnetic field in such systems. I will also discuss the identification of potential high field systems from a sample of CV candidates from the 5th data release of the LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) spectroscopic survey. I will conclude my talk by presenting an estimate for the magnetic field in LAMOST J003553.36+433341.4 from its LAMOST spectrum.

Verification of the Dynamically New Comets: Results from the N-body Simulation

Date: 2025-04-24
Speaker: Goldy Ahuja
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Comets are the preserved minor bodies which holds the primitive information about the early solar system. These bodies come into the inner solar system from two reservoirs, i.e., Kuiper Belt and Oort Cloud. Kuiper Belt is the source of Short-Period, low-inclination comets (SPCs) whereas Oort Cloud is the source of Long-Period Comets (LPCs) having isotropic inclination. Dynamically New Comets (DNC) are a long-period comets, with a semi-major axis > 10000 AU, entering the inner Solar System for the first time, which gives us an excellent opportunity to study their composition and origin. In this talk, I will explain the early classification for comets based on the basis of different dynamical and chemical properties. I will then explain the origin of a few long-period comets using the N-body dynamical simulation package, REBOUND. I will explain the N-body simulation code and benchmarking test with the well known asteroid, APOPHIS. I will conclude my talk with the inclusion of Non-gravitational forces and the difference with the results.

Advances in Direct Imaging of Exoplanets

Date: 2025-04-23
Speaker: Dr. Prashant Pathak
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

Direct imaging of rocky exoplanets remains a major scientific objective for current and next-generation large telescopes. While existing facilities have successfully imaged young, Jupiter-mass exoplanets at wide separations (>0.1"), detecting smaller, rocky planets poses a significantly greater challenge due to the stringent contrast requirements. The mid-infrared (mid-IR) regime offers a promising solution, providing optimal planet-star contrast for detecting thermal emissions from exoplanets in our solar neighbourhood. In this talk, I will discuss the key techniques enabling direct imaging of exoplanets, with a focus on two mid-IR high-contrast imaging (HCI) instruments: NEAR (New Earths in the Alpha Cen Region) and METIS (Mid-infrared ELT Imager and Spectrograph). The NEAR experiment demonstrates the feasibility of HCI at ten microns, achieving sub-mJy sensitivity within a few hours—sufficient to detect multiple Jupiter-mass planets around nearby stars. I will present its operational principles and scientific outcomes. Furthermore, I will explore METIS, the first-generation instrument for the Extremely Large Telescope (ELT), highlighting its HCI capabilities and expected performance.

Origin of Soft excess in Mrk50

Date: 2025-04-22
Speaker: Narendranath Layek
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

Active galactic nuclei (AGNs) are the most luminous and energetic sources in the universe, powered by the accretion of matter onto supermassive black holes (SMBHs) located at the centers of host galaxies. The soft excess refers to an enhancement of flux in the soft X-ray range (belowâˆ¼2 keV) over the primary power-law continuum, commonly observed in most Seyfert 1 AGNs. Its origin is a long-standing and unsolved puzzle in AGN studies. To investigate this, we conducted an extensive temporal and spectral analysis of the Seyfert 1 AGN Mrk 50, utilizing the observations from XMM, Swift, and NuSTAR. Two possible physical scenarios explain the origin of soft excess in AGNs , Warm Comptonization and Reflection from the ionized accretion disk. Both physical models successfully explained this behaviour in Mrk 50. Further, we investigated the origin using a model-independent approach using cross-correlation analysis between two X-ray bands (soft and hard) to examine the correlations and delays between them. In this seminar, I will present a detailed study of the origin of the soft excess in Mrk 50, based on our temporal and spectral analysis.

Investigating the explosion and progenitor properties of Type II core-collapse supernovae

Date: 2025-04-17
Speaker: Dr. Bhavya Ailawadhi
Venue: Online:https://imeet.vconsol.com/join/9563852195?be_auth=NDU5MzEx

Abstract

This presentation highlights the core components of my doctoral research, which focused on the photometric and spectroscopic characterisation of Type II core-collapse supernovae (CCSNe) and the development of automated data processing tools for time-domain surveys. CCSNe are end stages of massive stars, contributing to the chemical enrichment of the interstellar medium and influencing galaxy evolution through the dispersal of heavy elements. I conducted detailed observational studies of two transitional SNe—2020aze and 2020jfo—which do not fit cleanly into the classical Type IIP or Type IIL categories. SN 2020jfo exhibited a shorter plateau duration but shared key properties with Type IIP SNe. At the same time, SN 2020aze displayed characteristics that were more aligned with Type IIL SNe, including early flash features and a faster decline during the photospheric phase. These analyses contribute to the growing understanding that Type IIP and IIL SNe form a continuum rather than distinct classes. To investigate te role of progenitor environments, I conducted a statistical study on a broader sample of SNe by estimating metallicities using Fe line equivalent widths. This analysis provided insights into how progenitor metallicity influences light curve morphology and explosion characteristics. Complementing the observational work, I developed a fully automated aperture photometric pipeline in Python for the 4.0m International Liquid Mirror Telescope (ILMT). The pipeline processes time-delay integration (TDI) mode data, performs astrometric calibration, computes instrumental magnitudes, and derives calibrated light curves using catalogues like Pan-STARRS and SDSS. The pipeline is fully functional and is being used to derive long term light curves of variable sources.

Delving into the Extremes of Neutron Stars: Insights from Thermonuclear X-ray Bursts

Date: 2025-04-08
Speaker: Dr. Gaurava Kumar Jaiswal
Venue: Online:https://imeet.vconsol.com/join/2150369168?be_auth=NTI3ODEz

Abstract

Neutron stars (NSs) are ultra-dense remnants of massive stars, characterized by immense gravitational fields, temperatures, and densities, making them unique laboratories for studying matter under extreme conditions. Type-I X-ray bursts, observed from accreting NSs in low-mass X-ray binary systems, provide valuable insights into these environments. These bursts are driven by unstable thermonuclear burning of accreted hydrogen and/or helium on the NS surface, typically lasting from tens to hundreds of seconds, depending on the fuel composition. About 20% of these bursts are energetic enough to temporarily lift the NS photosphere by tens to hundreds of kilometers. Additionally, the nuclear burning during the bursts leads to the synthesis of elements as heavy as those in the Sn-Sb-Te mass region. Studying these events offers critical information about nuclear processes, burst-accretion disk interactions, and provides important constraints on NS properties such as spin and compactness. Observations from the Neutron Star Interior Composition Explorer (NICER) have advanced our understanding by offering unprecedented timing and good spectral sensitivity, enabling detailed studies of X-ray bursters. This talk will focus on the latest NICER findings, highlighting the complex interplay between X-ray bursts, accretion dynamics, and nucleosynthesis, and how these observations can help constrain the equation of state of NSs - the "holy grail" of NS physics.

Towards a Unified Understanding of Accreting Compact Objects

Date: 2025-04-07
Speaker: Dr. Aru Beri
Venue: Online:https://imeet.vconsol.com/join/8175021695?be_auth=MzAwMTg1

Abstract

Accreting neutron stars and black holes in X-ray binaries are powerful laboratories for exploring extreme physical conditions such as strong gravity, dense matter, and intense magnetic fields. In this talk, I will present recent discoveries from fast X-ray timing and spectral observations, including coherent and incoherent variability that reveals the nature of the innermost regions of these systems. Drawing on results from the AstroSat survey, I will highlight the discovery of an intermittent accretion-powered millisecond X-ray pulsar (AMXP)—a transient source and potential gravitational wave emitter. I will also explore the role of transient events, such as outbursts, in advancing our understanding of accretion processes and how multi-wavelength campaigns help capture the dynamic behavior of both neutron star and black hole systems. Finally, I will discuss the potential of combining X-ray polarization with timing-spectral and multi-band observations to build a more complete picture of both persistent and transient accreting compact objects.

FIELD-ANGLE OPTIMIZED DESIGN FOR WIDE-FIELD IMAGING X-RAY TELESCOPES

Date: 2025-04-03
Speaker: Mr. Neeraj K. Tiwari
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

Wide field of view (FOV) imaging X-ray telescopes play a crucial role in addressing some of the most challenging and unresolved questions in modern astrophysics. For example, they are crucial for probing the early formation of supermassive black holes (SMBH), rigorously testing the hypothesis that nanoflares are the primary mechanism sustaining coronal temperatures above million Kelvin, and detecting the electromagnetic counterparts of gravitational wave events. However, existing optical designs for X-ray telescopes, such as the Wolter type-1 (W1) and Wolter-Schwarzschild (WS) configurations, offer high angular resolution only along the optical axis and are therefore limited to narrow FOVs (a few arcminutes), while the scientific cases mentioned above require high angular resolution across a much wider FOV (up to 60 arcminutes). In this talk, I will introduce a new optical design, the field-angle optimized (FO) design, specifically developed for wide FOV X-ray imaging telescopes. I will discuss the methodology behind this design, compare its performance with existing optical designs, and explore its feasibility for implementation in wide FOV solar X-ray telescopes.

Time domain photometric study of peculiar Blazars

Date: 2025-04-01
Speaker: Dr. Shubham Kishore
Venue: Online: https://meet.google.com/ihi-bmxq-syc

Abstract

Blazars often exhibit random, aperiodic, and stochastic behaviours in their flux across all observational electromagnetic (EM) bands over a wide range of timescales. However, the underlying causes are not yet fully understood regarding which flux variations on the intra-day/day timescales are most poorly comprehended. These variations are primarily related to accretion or jet physics, as jets are powered by accretion. In the talk, I will elucidate my findings that include the quasi-periodic oscillatory signatures and flare episodes detected in three individual Blazar candidates observed with Transiting Exoplanet Survey Satellite (TESS).

The African Network of Women in Astronomy (AfNWA) and SciGirls: Examples of social activism

Date: 2025-03-20
Speaker: Dr. Mirjana Povic
Venue: Online: https://meet.google.com/cuw-pwht-zej

Abstract

The world faces a significant gender gap in science. In Africa, in average the population of female scientists is below 25%. The African Network of Women in Astronomy (AfNWA) is an initiative that aims to connect women (or people who identify as such) working in astronomy and related fields in Africa. It was established in September 2020 as one of the committees of the African Astronomical Society (AfAS). With AfNWA we want to ensure the future participation of girls and women at all levels in the development of astronomy and science in Africa. Our main goals are to improve the status of women in science in Africa and to use astronomy to empower girls and women, and to inspire more girls to pursue Science, Technology, Engineering and Mathematics (STEM). This talk will summarise the activities carried out by AfNWA and the current status of women in astronomy in Africa. It aims to give visibility to the work and achievements of the AfNWA community, and the various activities carried out across the continent to support girls and women living and working in under-represented communities through astronomy. In Ethiopia, only about 13% of all scientists are women, and this fraction is even lower when considering the fundamental sciences. Girls avoid choosing STEM mainly due to a lack of support and/or information. This becomes even more evident when going to remote areas, where 80% of the Ethiopian population lives. The SciGirls project aims to improve the gender gap in science in the long-term by empowering female secondary school students and their female science teachers who are working and living in remote and rural areas through astronomy and its multidisciplinarity. In 2022 and 2024 we organised a carefully designed capacity-building workshop for 60 participants across Ethiopia, with the aim of training future STEM advocates in rural and remote areas. The girls and teachers carried out different activities in their communities after the training. Most of participants came from the regions that have been severely affected by conflicts over the past 4 years. SciGirls is one of the 2022 and 2024 projects funded by OAD. During this talk, we will share valuable experiences we have gained through interaction with girls and female teachers who work and live in very harsh conditions, where they rarely have any external support to fulfill their dreams. The SciGirls approach has so far yielded very positive results and the project can serve as a model also in other countries.

Gamma-Ray Bursts (GRBs) as electromagnetic (EM) counterparts of Gravitational Wave (GW) sources

Date: 2025-03-06
Speaker: Dr. Suman Bala
Venue: Room No. 113, Thaltej

Abstract

Gamma-Ray Bursts (GRBs) are the brightest explosions in the Universe since the Big Bang. We have comprehensive knowledge about the GRBs, but there are many open questions even after fifty years of the first detected GRB, especially about the prompt emission phase. The detection of gamma-ray burst GRB 170817A by Fermi-GBM, coinciding with gravitational wave (GW) GW170817, is one of the extraordinary discoveries in the history of the multimessenger era. It is not only the first binary neutron star (BNS) merger detected by the advanced (LIGO-Virgo) GW detectors; it is the only GW detection with a confirmed electromagnetic (EM) counterpart. The Fermi Gamma-ray Burst Monitor (GBM) is an all sky monitoring instrument sensitive to photon energies from 8 keV to 40 MeV. Its capabilities makes it ideal for providing simultaneous gamma-ray observations of gravitational-wave transients. Fermi-GBM continues to look for similar multimessenger detections through on-board triggers as well as subthreshold searches for weak transients, performed both in high-time-resolution continuous data and in targeted follow-ups of gravitational-wave events. In this talk, I will present an overview of GRBs and recent results from targeted and subthreshold searches as a counterpart of GW events.

Hot Jupiter Exoplanets: The Enigmatic Giants of Astrophysics

Date: 2025-03-04
Speaker: Dr. Soumya Sengupta
Venue: Room No. 113, Thaltej

Abstract

Since the discovery of the first exoplanet, 51 Pegasi b, Hot Jupiters (HJs)—Jupiter-like exoplanets orbiting close to their host stars—have remained a central focus in exoplanetary science. Unlike planets in our solar system, these unique systems allow us to study them directly through their infrared emission. Due to intense stellar irradiation, Hot Jupiters exhibit extremely high temperatures, resulting in distinct emission spectra originating primarily from their day-side hemispheres, especially in tidally locked systems. Analyzing these emission spectra provides valuable insights into the temperature structure and chemical composition of these intriguing exoplanets. However, the overlap between planetary and stellar emissions poses a persistent challenge for planetary atmospheric modeling. Additionally, the strong day-night atmospheric flow, driven by the extreme temperature contrast between hemispheres, introduces variability in the observed emission spectra. Another intriguing feature of Hot Jupiters is their larger observed radii compared to Jupiter; a phenomenon known as the radius inflation problem. In this talk, I will explore these fascinating questions surrounding Hot Jupiters, using fundamental physics concepts to unravel the mysteries of these extraordinary worlds.

Fabry-Perot wavelength calibration system for precise radial velocity measurements

Date: 2025-02-13
Speaker: Shubhendra Nath Das
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

Although more than 5000 exoplanets have been detected, only a few have had their profiles constructed. Besides the radius, mass is significant variable as well, which makes the radial velocity (RV) method important. Yet, instruments and calibration of the equipment pose constraints when it comes to the detection of small Doppler shifts (sub-m/s). One such issue is the accuracy of the wavelength calibration itself. Addressing this, the cost-effective and stable Fabry-Pérot (FP) etalons are an alternative to uranium-argon (UAr) lamps because they last significantly longer and have a spectrum that is ideal for supporting better wavelength calibration for very precise RV measurements. We are changing PARAS-2’s calibration system from UAr to FP. In this seminar, we will give project progress report and show laboratory testing outcomes of the FP system.

Solar Coronal Phenomena: Imaging X-ray Spectroscopy

Date: 2025-02-11
Speaker: Dr. Biswajit Mondal
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

The Sun's outer atmosphere, known as the corona, is significantly hotter than its surface, presenting a long-standing scientific mystery. One hypothesis is that small, frequent bursts of energy, called nanoflares, may be responsible for this heating, though the exact mechanism remains unclear. Additionally, certain elements in the corona appear more abundant than expected, a phenomenon termed the "FIP effect," which might also be linked to coronal heating processes. Imaging X-ray spectroscopy offers a powerful method for investigating these solar mysteries. In this talk, we will explore these intriguing questions about the Sun and discuss how imaging X-ray spectroscopy can provide insights. We will introduce the Marshall Grazing Incidence X-ray Imaging Spectrometer (MaGIXS) sounding rocket experiment and its recent successful flight, designed to probe these enigmatic aspects of the Sun.

Diffuse Interstellar Bands in the Milky Way as seen by GAIA

Date: 2025-02-04
Speaker: Mathias Schultheis
Venue: Room No. 113, Thaltej

Abstract

Diffuse interstellar bands (DIBs) are interstellar absorption features originating from the interstellar medium, quasi-consensually attributed to large organic molecules. DIBs exist in the optical and in the infrared. Most of the DIBs show a tight relation with interstellar reddening, and can therefore be used as an excellent tracer of the ISM. Beside the equivalent width of the DIBs, radial velocities profiles can be derived and be used to study e.g the Galactic rotation curve of the carrier. I will present the capacity of the Gaia-Radial Velocity Spectrometer (RVS) in Gaia DR3 to reveal the spatial distribution of the unknown molecular species responsible for the most prominent DIB at 862\,nm in the RVS passband exploring the Galactic interstellar medium within a few kiloparsecs from the Sun. Nearly 500.000 DIB measurements have been obtained in a homogeneous way covering the entire sky, making it the largest sample of DIB measurements so far. I compare spatial distributions of the DIB carrier with interstellar reddening and find evidence that DIB carriers are present in a local bubble

Tracing Cosmic Origins: Unveiling Element Formation Through Stellar Archaeology

Date: 2025-01-09
Speaker: Pallavi Saraf
Venue: Online: https://imeet.vconsol.com/join/8509176600?be_auth=NjMwMzQ4

Abstract

The origins and distribution of chemical elements in the Universe have long been a subject of investigation, with many unresolved questions remaining. The oldest stars in our Milky Way are rare relics from the early Universe, preserving the chemical imprints of the first stars and supernova explosions. These stars are crucial in addressing questions about element formation processes that occurred around 13 billion years ago. I will explain on how I employ "Stellar Archaeology": the use of observations and analysis of the chemical properties of the oldest stars in Galaxy, to answer outstanding questions about the early Universe and the origins of the chemical elements in the Cosmos. One of the significant unanswered questions in astrophysics is the site of the rapid neutron-capture process (r-process). While the optical counterpart AT 2017gfo of the kilonova GW 170817 did provide evidence of the r-process in neutron star mergers, important details are still unsolved. Neutron star mergers alone seem to be unable to explain r-process enrichment in the Universe, and there are still open questions with respect to their time scale. I will discuss some of the results from observations of r-process stars with the Gran Telescopio Canarias (GTC) and the Very Large Telescope (VLT), as well as CEMP-r/s stars observed with the KECK telescope and VLT. Additionally, I will share findings from the HESP-GOMPA survey conducted by our group. Finally, I will discuss how my expertise aligns with the facilities at the Physical Research Laboratory (PRL), such as PARAS-2.

Probing accretion process and emission mechanism of X-ray pulsars in multi-wavelength

Date: 2025-01-08
Speaker: Manoj Mandal
Venue: Online: https://imeet.vconsol.com/join/8880252012?be_auth=MjU0NzIy

Abstract

The timing and spectral studies have been carried out for several X-ray pulsars to probe the emission mechanism, accretion process, and spectral states. The timing and spectral properties evolve significantly above the critical luminosity. The accretion mode, beaming patterns, and emission mechanism evolve significantly above this luminosity. A significant evolution of temporal and spectral properties is observed during the state transition for X-ray pulsars, 1A 0535+262 and RX J0440.9+4431. A variable cyclotron line was detected from 1A 0535+26, and the magnetic field was estimated using the cyclotron line energy. The variation of the cyclotron line is probed. The significant evolution of line energy with luminosity was observed, which may be linked with the transition of state in the X-ray pulsars. The unstable burning of accreted material on the surface of neutron stars induces thermonuclear (Type-I) bursts. Thermonuclear bursts can be used to probe several properties of neutron stars. Multiple thermonuclear bursts were detected from MAXI J1816−195 and Aql X-1. The details of timing and spectral properties are studied during the X-ray bursts. The estimated mass accretion rate indicates the stable burning of hydrogen via a hot CNO cycle during the bursts.

Probing the Cold Molecular Gas in Luminous Dusty Star-forming Galaxies at z~1-6

Date: 2025-01-02
Speaker: Ms. Prachi Prajapati
Venue: Seminar Room No. 113, Thaltej

Abstract

The galaxies and star formation we see at present are attributed to a long history of galaxy formation and evolution. Reconstructing back in time the physical processes that led to the existing galaxies and explaining them in terms of different properties of the matter is one of the prime goals of the observational cosmology. In particular, studying the molecular gas content of high-redshift dusty star-forming galaxies (DSFGs) is of utmost importance for observationally confirming the galaxy formation and evolution theories. Observing the gas with low excitation leads to better mass estimates and also helps in deriving the gas and dust properties of these galaxies more accurately. Having a large and diverse sample of DSFGs for such a study plays an important role in setting up statistically significant trends within the DSFG population and in determining whether or not there are significant differences in the gas properties of DSFGs compared to other populations. This talk will be focused on the VLA large program to observe CO (1–0) in high-redshift DSFGs (0.8 < z < 6.5) for deriving insights on the cosmic star formation history.

2024

The Journey of Star Formation: From Collapsing Cloud to Accreting Protostar

Date: 2024-12-26
Speaker: Kushagra Srivastav
Venue: Seminar Room No. 113, Thaltej

Abstract

Star formation is a fundamental and vital process of molecular cloud evolution and the creation of stellar systems. Stars are born within the dense, compact, and low-temperature regions of molecular clouds called cores. These dense cores are the sites that eventually lead to the formation of a protostar, surrounded by a protoplanetary disc and an infalling envelope. Understanding the various stages, from the collapsing cores to the formation and growth of a protostar, requires understanding the physical and dynamic processes involved. In this talk, I will discuss the various stages of star formation and the role of different physical processes involved and also touch upon the evolution of a protoplanetary disk and the accretion from the disk to the star.

Dynamics of Solar Corona Heliospheric Interaction

Date: 2024-12-19
Speaker: Ritik Dalakoti
Venue: Room No. 113, Thaltej

Abstract

The lower solar corona exhibits significant dynamic activity across various scales, with magnetic features continuously forming and disappearing on the Sun's surface. Understanding the relationship between these features and the heliosphere is crucial for understanding the heliosphere and predicting the flow of plasma within it. This knowledge is essential for forecasting space weather accurately. In this study, we investigate an M-class solar flare from which plasma eventually propagated into the heliosphere. We examine the evolution of First Ionization Potential (FIP) elements during the flare using the soft X-ray spectrum from the Solar X-ray Monitor aboard Chandrayaan-2. Additionally, we analyse in situ particle data to determine whether the evolution of FIP elements can be observed at 1 AU or other heliospheric locations.

Uncovering the hidden physical structures and protostellar activities in the Low-Metallicity S284-RE region: results from ALMA and JWST

Date: 2024-12-12
Speaker: Omkar Jadhav
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Over the past decade, Spitzer and Herschel observations have provided invaluable opportunities to explore key structures in star-forming regions, such as bubbles and filaments. With the advent of the James Webb Space Telescope (JWST) and the Atacama Large Millimeter/submillimeter Array (ALMA), researchers have access to a powerful multi-wavelength approach, which can be used to investigate the dust and gas structures around embedded protostars in exceptional detail and to study distant star-forming sites. We present an observational study of the S284-RE region (d ~5.0 kpc), a low-metallicity area associated with the extended S284 HII region. The S284 region is characterized by features such as filaments, pillars, and globules, which are hallmarks of star-forming regions shaped by stellar feedback. Multi-scale and multi-wavelength datasets are used to explore the underlying physical structures and protostellar activities in this site. In this talk, I will provide a detailed overview of our findings and their implications for understanding the physical processes driving star formation in S284-RE.

TOI-6038 A b: Discovery of a sub-Saturn orbiting a late F-type star in a wide binary system

Date: 2024-11-28
Speaker: Sanjay Baliwal
Venue: Room No. 113/114, Thaltej

Abstract

The discovery of exoplanets has revealed numerous planetary populations with no analogs in the Solar System. Among these, sub-Saturn exoplanets, larger than Neptune but smaller than Saturn and in close orbits around their host stars, represent a particularly intriguing class. Sub-Saturns are often described as failed gas giants, possessing equally massive cores but significantly smaller total masses due to their much smaller accreted envelopes. In this talk, I will discuss our recent discovery of a sub-Saturn TOI-6038 A b using PARAS-2 spectroscopic observations. TOI-6038 A b is a relatively dense sub-Saturn with a mass of approx 79 Earth masses and a size of about 6.4 Earth radii, orbiting a bright, metal-rich late F-type star in a nearly circular orbit with a period of about 5.83 days. The system also contains a wide-orbit binary companion, TOI-6038 B, which is an early K-type star at a projected separation of 3217 AU. Internal structure modeling indicates that about 74% of the planet’s mass is made up of rocky materials forming the core, with the rest consisting of a low-density H/He envelope. TOI-6038 A b lies in the transition zone between the Neptunian ridge and savanna, making it a key system for understanding the formation and evolution of close-in sub-Saturns. While its high density is consistent with planets shaped by high-eccentricity tidal migration, the exact migration pathway remains unclear. The planet’s bright host star makes it a promising candidate for future atmospheric escape and orbital architecture observations, which will help us better understand its overall evolution.

Active Galactic Nuclei - An Overview

Date: 2024-11-14
Speaker: Priyadarshee P. Dash
Venue: Seminar Room #113 (Thaltej Campus)

Abstract

Active Galactic Nuclei (AGN) are among the most energetic and luminous astronomical sources in the Universe. With bolometric luminosities reaching up to Lbol = 1048 erg s−1 (or 1015 L⊙), the AGNs emit across the entire range of the electromagnetic spectrum. The central source in an AGN is thought to be a supermassive black hole actively accreting nearby matter through an accretion disk, releasing substantial energy in the process. In this talk, I will provide an overview of various components of an AGN that account for its distinctive features across different regions of the electromagnetic spectrum. Emphasis is placed on the X-ray spectrum of the AGN, which offers crucial insights into the energetic processes close to the central engine. Finally, I will discuss the Unified Model of AGNs, which explains how diverse astronomical objects - such as Seyfert galaxies, quasars, radio galaxies - fit under the overarching category of AGN, despite differing in their observational characteristics.

Enhancing Localization of Daksha-GRBs using Coded Mask Imaging Technique

Date: 2024-10-24
Speaker: Mr. Ashish Kumar Mandal
Venue: Seminar Room #113 (Thaltej Campus)

Abstract

Gamma ray bursts (GRBs) are the intense extragalactic gamma ray flashes which last for a few milliseconds to a few hundreds of seconds. The simultaneous detection of both the gravitational wave (GW) and electromagnetic (EM) signals is the only way to probe the origin of short GRBs (flash duration<2s). Hence both sensitivity of GW and EM detectors is essential in addition to all sky coverage of EM detectors. Daksha is a proposed high energy transient mission of India providing full sky coverage with better sensitivity whose primary goal is to detect electromagnetic counterparts of gravitational wave events. In addition to the sensitivity of EM detectors, precise localisation of events is very crucial for their follow up afterglow study in other EM wave bands. The short GRB afterglow study is important since it carries signatures of elements heavier than iron formed from r-process nucleosynthesis of neutrons in the ejecta of BNS merger, also many fundamental physics can be verified from the signatures carried by these events at high redshifts. The proposed Daksha configuration estimates localization based on the ratio of photon counts on detectors at different orientations. In this work we explore the improvement of localization in Daksha if a coded mask is used for either low or medium energy detectors. For this, we have estimated both sensitivity and localization accuracy of Daksha detectors.

Stellar evolution in star clusters

Date: 2024-10-17
Speaker: Dr. Ranjan Kumar
Venue: Seminar Room #113 (Thaltej Campus)

Abstract

Star clusters are ideal places to explore the formation and evolution of stellar systems. They host stars of almost all the evolutionary phases. Mostly, young and massive stars dominate the open clusters, whereas the globular cluster traces the evolutionary sequence of low-mass stars. They also contain exotic stellar populations (e.g., blue straggler stars, blue hook stars, AGB-manque stars, low mass He white dwarf, and catalysis variables, etc.), which are byproducts of dense stellar environments and/or binary stellar systems in the cluster. The ultraviolet (UV) emission in star clusters is mostly dominated by the young (massive), exotic, and evolved stars, which are important to studying the formation and evolution of a star cluster. I will present the first comprehensive ultraviolet (UV) source catalogue (UVIT DR1) of UV photometry in various FUV (1300−1800 A) and NUV (2000−3000 A) filters of the UltraViolet Imaging Telescope (UVIT) onboard AstroSat. UVIT DR1 includes a total of 239,520 unique UV-bright sources, of which 70,488 sources have FUV magnitudes, and 211,410 have NUV magnitudes covering a sky area of ~ 58 sq. deg. I will showcase the results of a newly discovered hot post-AGB star in a sparse Galactic globular cluster, E3 (ESO 37-1), using UVIT observations. I will discuss its binarity, chemical abundance and evolutionary status utilizing observed/archival datasets of the UVIT/AstroSat, Gaia DR3, and high-resolution spectra. In this talk, I will also discuss the preliminary results of the study of blue straggler stars in selected star clusters using photometric and spectroscopic observations of the 2.5m telescope, Mt. Abu.

Gravitational wave and multi-messenger signals from compact binary mergers

Date: 2024-10-10
Speaker: Prof. Kunal Mooley
Venue: Seminar Room #113 (Thaltej Campus)

Abstract

More than 150 high-significance gravitational wave (GW) sources, primarily binary black hole mergers, have been detected by ground-based GW observatories to date. The joint GW and EM detections of the binary neutron star merger GW170817 yielded a scientific bonanza in fields as wide-ranging as gravitational physics, nucleosynthesis, neutron star equation of state, relativistic explosions and jets, and cosmology. The EM counterpart of GW170817 gave insight into the mass, speed and composition of the slow-moving merger ejecta through the rapidly-evolving kilonova emission (from r-process nucleosynthesis), while the long-lasting afterglow probed the energetics and morphology of the relativistic ejecta (jet and cocoon). It also provided an unambiguous link between neutron star mergers and short gamma-ray bursts, and facilitated a precise measurement of the Hubble's constant. I will discuss the astrophysics and fundamental physics learned from GW170817 and what could be learned from future GW events: like NS-NS and NS-BH mergers. I will also describe the searches for systems like NS-BH and high-mass-ratio systems in the Galactic Center that will be important for space-based detectors like LISA.

Resonant and Secular Evolution of Three Body Systems – With Applications to Planetary Systems and Gravitational Wave Sources

Date: 2024-09-26
Speaker: Dr. Hareesh Gautham Bhaskar
Venue: Seminar Room #113 (Thaltej Campus)

Abstract

The three-body problem describes the motion of three objects under the influence of their mutual gravity. It is one of the oldest problems in astrophysics. No closed form analytical solution is possible to the general three body problem due to its chaotic nature. Despite its rich history, the three-body problem remains an active area of research. Recent advancements in the field are motivated by new discoveries pertaining to exoplanets and blackholes. In this work we focus on resonant and secular interactions in three body systems, with a goal to understand how these interactions influence the long-term stability and evolution of three body systems. We apply our theoretical investigations of these physical processes to a wide range of observed systems. We find that secular and resonant perturbations can significantly affect the stability limit of mutually inclined planets. Planets in retrograde configurations are much more stable than prograde configurations, with secular perturbations significantly destabilizing the system when 200 < Imut < 1600. In addition, we find that secular three body dynamics can also help us constraint the inclination of the hypothetical Planet-9 in the outer solar system, and explain the observed retrograde stellar obliquities of hot Jupiters. We also propose a novel pathway through which compact binaries could merge due to eccentricity excitation caused by resonant interactions induced by a massive coplanar companion. Specifically, a compact binary migrating in an AGN disk could be captured in a precession-induced resonance, when the apsidial and nodal precession rates of the binary are commensurable to the orbital period around the supermassive black hole. Eccentricity is excited when the binary sweeps through the resonance which happens only when it migrates on a timescale 10-100 times the libration timescale of the resonance. The eccentricity excitation of the binary can reduce the merger timescale by a factor up to 103 -105.

Contact Binaries: A Detailed Analysis Through Photometric and Spectroscopic Data

Date: 2024-09-19
Speaker: Dr Alaxender Panchal
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Eclipsing binaries (EBs) are essential for understanding stellar evolution, providing direct measurements of stellar masses, radii, and luminosities. Among these, W UMa-type systems, also known as contact binaries, are particularly important because the stars share a common envelope, leading to complex energy exchanges. W Uma-type systems are important as they give information about component interactions, mass-trasfer between components, period evolution, extra bodies around inner binary etc. In this talk, I will present a long-term photometric and spectroscopic analysis of four W UMa-type systems. This study combines light curves with spectroscopic data, revealing details about their orbital parameters, period changes, and evolution. The findings shed light on the internal dynamics of contact binaries, offering valuable insights into stellar interaction and evolution. I will discuss the methods used, the findings, and their significance in the broader context of contact binaries.

Assembly-Integration-Testing of ProtoPol, its on-sky commissioning, and subsequent status of its data reduction pipeline

Date: 2024-08-01
Speaker: Arijit Maiti
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

ProtoPol is a medium-resolution echelle spectro-polarimeter initially conceived as the prototype instrument of the currently under development M-FOSC-EP (Mt. Abu Faint Object Spectrograph and Camera-Echelle Polarimeter) instrument – a two-channel multimode instrument which is currently being designed for PRL 2.5m telescope at Mt. Abu. Though ProtoPol was initially conceived to evaluate the development methodology of M-FOSC-EP using commercially available off-the-shelf components, it was later elevated to the level of a full-fledged back-end instrument for PRL telescopes. ProtoPol was designed on the concept of echelle and cross-disperser gratings to record the cross-dispersed spectra in the wavelength range from 390 to 940 nm with a resolution in the range of 7000-8000. ProtoPol has been successfully developed and commissioned on PRL 1.2m and 2.5m telescopes since December 2023, and a variety of observations are being carried out for instrument characterization and scientific purposes. In this talk, I shall discuss the features and properties of ProtoPol, the assembly-integration-testing of the instrument in the laboratory, and its subsequent commissioning on PRL telescopes for on-sky characterization and science observations. I will also talk about the current status of the data reduction pipeline being developed for ProtoPol.

FiberPol-6D- Spectropolarimetric Integral Field mode for the SAAO 1.9 m Telescope using fibers

Date: 2024-07-29
Speaker: Dr Siddharth Maharana
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

Most optical spectropolarimeters built to date operate as long-slit or point-source instruments; they are inefficient for observations of extended objects such as galaxies and nebulae. 2D spectropolarimetry technique development is a major challenge in astronomical instrumentation. At the South African Astronomical Observatory (SAAO) FiberLab, we are developing a spectropolarimetry capable Integral Field front-end called FiberPol(-6D) for the existing SpUpNIC spectrograph on the SAAO 1.9 m telescope. SpUpNIC is a general purpose 2 arc-minute long-slit spectrograph with a grating suite covering the wavelength range from 350 to 1000 nm. FiberPol generates 6D observational data: x-y spatial dimensions, wavelength, and the three linear Stokes parameters I, q and u. Using a rotating half-wave plate and a Wollaston prism, FiberPol executes two-channel polarimetry, and each channel is fed to an array of 14 fibers, corresponding to a field of view of 10×20 arcseconds^2 sampled with 2.9 arcsecond diameter fiber cores. FiberPol aims to achieve a polarimetric accuracy of 0.1 % per spectral resolution bin. Further, it can also function as a non-polarimetric integral-field unit. The primary science goals include study of ISM of nearby galaxies to test the models of dust grain alignment and its dependence with the ambient magnetic field. The instrument design has been completed and it is currently being assembled and characterized in the lab. It is scheduled for on-sky commissioning in the second half of 2024. In this talk, I will present the scientific and technical goals of FiberPol, its overall design and initial results from the lab assembly and testing. FiberPol is a low cost technology demonstrator for 10m SALT and other large telescopes such as the 30m class telescopes. It can be modified and replicated for use on any existing spectrograph, especially on bigger telescopes.

Multi-wavelength study of Blazars

Date: 2024-07-18
Speaker: Dr. Avik Kumar Das
Venue: Online : https://imeet.vconsol.com/join/5555049068?be_auth=NzQwNjQ0

Abstract

Blazars are a special subclass of active galactic nuclei, with relativistic jets pointing close to observers’ line of sight, making them the most luminous and rapidly variable extra-galactic sources in the universe. Their observed emissions are highly Doppler boosted and observable across the entire accessible electromagnetic spectrum (from radio to gamma-rays), with diverse variability timescales ranging from minutes to years. The observed emissions imply extreme physical conditions that are beyond replication by any current or future terrestrial laboratory. Almost every aspect, from jet formation, collimation to gamma-ray production in the jets, is poorly understood. However, over the past decade, the availability of long-term, high-cadence data in the multi-wavelength regime (i.e., optical, X-ray, and gamma-ray) has been helpful in addressing some aspects of these issues through study of i) multi-wavelength flux and spectral variability on diverse time scales, ii) physical origin of quasi periodic oscillations (QPOs) observed in the light curves, iii) multi-wavelength flux distribution of Blazars. In this talk, we will discuss our recent interesting results on the above topics related to Blazar emission properties.

Precision in Motion: Fabry-Perot Etalon as a Wavelength Calibrator for Extreme Precision Radial Velocity Methods

Date: 2024-06-20
Speaker: Shubhendra Nath Das
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

To date, over 5500 exoplanets have been observed, with the radial velocity method, utilizing Doppler shifts in light, proving its effectiveness. Advancements in spectrograph technology now enable us to detect and analyze subtle Doppler signals, potentially leading to the discovery of Earth-like exoplanets. However, challenges remain, particularly in wavelength calibration techniques. To address this, we propose a new approach utilizing Fabry-Perot etalons (FPE) in conjunction with PRL's cutting-edge spectrograph, PARAS-2. In this presentation, we will discuss recent advancements, including calculations regarding pressure and temperature stability, and showcase the design of the optomechanical assembly for the FPE.

Discovery and Characterization of a Dense Sub-Saturn TOI-6651b

Date: 2024-06-10
Speaker: Sanjay Baliwal
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The sub-Saturn classification of exoplanets refers to planets larger than Neptune but smaller than Saturn, typically falling within the range of 4-8 Earth radii. Sub-Saturns are considered failed gas giants with equally massive cores but significantly smaller total masses, having accreted envelopes that are much less than their cores. The absence of these planets in our solar system highlights the variety of possible planetary systems, and studying them around other stars provides valuable insights into this diversity. In this talk, I will discuss our recent discovery of a sub-Saturn TOI-6651b using PARAS-2 spectroscopic observations. TOI-6651b is transiting around a sub-giant, metal-rich G-type star in a ~5.06 day orbit. Joint fitting of the radial velocities from PARAS-2 and transit photometric data from TESS revealed planetary mass of ~59.4 Earth masses and a radius of ~5.30 Earth radii. TOI-6651b has a bulk density of ~2.18 g cm-³, positioning it among the select few known dense sub-Saturns and notably the densest among those detected with TESS. We find that a considerable portion ~85% of the planet's mass consists of dense materials such as rock/iron in the core, while the remaining mass comprises a low-density envelope of H/He. The existence of TOI-6651b challenges conventional planet formation theories and could be a result of merging events or significant atmospheric mass loss through tidal heating.

Understanding the Role of magnetic fields in the G47 filamentary cloud

Date: 2024-06-06
Speaker: Omkar Jadhav
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Several observational studies have shown that filaments are active sites of star-formation which makes them ideal laboratories to study the physical processes involved. However, star-formation remains a highly complex process, which is driven by an interplay between gravity, turbulence, magnetic fields, and stellar feedback. Among these, the exact role of magnetic fields in the process of star-formation and its interplay with other factors is the least understood. In this talk I will try to address two main questions: What role do magnetic fields play in the star formation process in filaments? How does stellar feedback affects the morphology of magnetic fields in filaments? Subsequently, I will present findings from our ongoing research on the factors responsible for driving the star-formation in the G47 filamentary cloud.

CHALLENGES INVOLVED IN THE DEVELOPMENT OF X-RAY ASTRONOMY TELESCOPES

Date: 2024-05-30
Speaker: Neeraj K. Tiwari
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The advancement in X-ray astronomy is directly linked to the high-sensitivity observations primarily provided by X-ray telescopes. The fundamental parameters of X-ray telescopes, such as effective area, angular resolution, field of view, and energy response, define observational sensitivity. Significant improvement has been achieved in these parameters over the past few decades. However, further enhancement from a basic design perspective is constrained by the fact that X-ray reflection occurs only at grazing incident angles. This limitation leaves only the option for further improvement by reducing the geometrical uncertainty associated with X-ray mirrors. In this talk, I will discuss the role of X-ray telescopes in X-ray astronomy, their working principles, the possibility of improvement through design, various fabrication methods, metrology, along with their limitations.

On the nature of Hub-Filament Systems in Galactic "Snake" IRDC G11.11-0.12

Date: 2024-05-28
Speaker: Dr. Naval Kishor Bhadari
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Hub-Filament Systems (HFSs) are complex, web-like structures of filaments within molecular clouds and are widely known as nurseries for massive stars. While there is evidence of mass transfer from the molecular clouds to the hub through filaments, the exact driving factors of this process remain poorly understood. In this talk, I will review the current understanding of HFSs and present the findings from our ongoing work on the Galactic 'Snake' IRDC G11.11-0.12, which is known to host multiple HFSs. I will conclude by discussing how HFSs act as efficient material collection systems, ultimately favouring the formation of star clusters, including massive stars.

Impact of mass transfer rate on the behaviour of cataclysmics

Date: 2024-05-09
Speaker: Aakash
Venue: Seminar Room #113/114 (Thaltej Campus)

Abstract

The transfer of mass between the two stellar components via an accretion disc, a characteristic of cataclysmic variables (CVs), gives rise to various intriguing astrophysical phenomena such as nova, dwarf nova, etc. It appears that the rate at which this transfer of mass takes place plays a significant role in the dwarf nova cycle and the behaviour of cataclysmics in general. Depending on it, a CV may show characteristics of a dwarf nova with semi-regular outbursts or a novalike variable. The canonical evolution model of cataclysmics suggests that different mechanisms are responsible for driving the mass transfer above and below the period gap (2 hr ≲ Porb ≲ 3 hr). However, the reduction and enhancement in the mass transfer rate (e.g. novalikes) and the underlying causes are still not understood well. In this talk, I will discuss the response of a cataclysmic system to different mass transfer rates. I will also present the calculation of mass transfer rate for the long period dwarf nova system V1948 Cyg and discuss how the results accord with the disc instability model.

Daksha: Indian Eyes on Transient Skies

Date: 2024-04-15
Speaker: Prof. Varun Bhalera
Venue: Seminar Room #113/114 (Thaltej Campus)

Abstract

Daksha is a proposed High Energy transients mission that will have higher sensitivity than any other mission in the world. Daksha will comprise of two satellites covering the entire sky from 1 keV to > 1 MeV. The primary objectives of the mission are to discover and characterize electromagnetic counterparts to gravitational wave sources; and to study Gamma Ray Bursts (GRBs). With its broadband spectral response, high sensitivity, and continuous all-sky coverage, it will discover fainter and rarer sources than any other existing or proposed mission. Daksha can make key strides in GRB research with polarization studies, prompt soft spectroscopy, and fine time-resolved spectral studies. In addition, Daksha is a versatile all-sky monitor that can address a wide variety of science cases. Daksha will provide continuous monitoring of X-ray pulsars. It will detect magnetar outbursts and high energy counterparts to Fast Radio Bursts. Using Earth occultation to measure source fluxes, the two satellites together will obtain daily flux measurements of bright hard X-ray sources including active galactic nuclei, X-ray binaries, and slow transients like Novae. Correlation studies between the two satellites can be used to probe primordial black holes through lensing. Daksha will have a set of detectors continuously pointing towards the Sun, providing excellent hard X-ray monitoring data. Closer to home, the high sensitivity and time resolution of Daksha can be leveraged for the characterization of Terrestrial Gamma-ray Flashes. In this talk, I will discuss the scientific impact of Daksha in all these areas

Cloud-Cloud Collision: Formation of Hub-Filament Systems and Associated Gas Kinematics

Date: 2024-04-12
Speaker: Arup Kumar Maity
Venue: Seminar Room #113/114 (Thaltej Campus)

Abstract

Massive star-forming regions (MSFRs) are commonly associated with hub-filament systems (HFSs) and sites of cloud-cloud collision (CCC). Recent observational studies of some MSFRs suggest a possible connection between CCC and the formation of HFSs. To understand this connection, we analyzed the magneto-hydrodynamic simulation data from Inoue et al. (2018). This simulation involves the collision of a spherical molecular cloud with a plane-parallel sea of dense molecular gas at a relative velocity of about 10 km/s. Following the collision, turbulent and non-uniform cloud undergoes shock compression, rapidly developing filamentary structures within the compressed layer. We found that CCC can lead to the formation of HFSs, which is a combined effect of turbulence, shock compression, magnetic fields, and gravity. The collision between the cloud components shapes the filaments into a cone and drives inward flows among them. These inward flows merge at the vertex of the cone, rapidly accumulating high-density gas, which can lead to the formation of massive star(s). The gas distribution in the position-velocity and position-position spaces highlights the challenges in detecting two cloud components and confirming their complementary distribution if the colliding clouds have a large size difference. However, such CCC events can be confirmed by the position-velocity diagrams presenting gas flow toward the vertex of the cone, which hosts gravitationally collapsing high-density objects, and by the magnetic field morphology curved toward the direction of collision. In this talk, I will present these initial results.

Optical Monitoring of a long-period dynamically new comet C/2020 V2 (ZTF)

Date: 2024-04-04
Speaker: Goldy Ahuja
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Comets are the pristine bodies that retain the essential information of the early Solar System. They are mainly placed in two reservoirs, i.e., Kuiper Belt and Oort Cloud. These reservoirs hold a large number of comets. The comets in the Kuiper belt are responsible for the short-period low-inclination known as Jupiter family comets (JFC), while comets in the Oort Cloud account for the long-period comets with different inclinations (as compared to the JFC). Studying the composition of different classes of comets is essential to understanding the formation and evolution of the Solar System. Dynamically New Comet (DNC) is a subcategory of long-period comets with a semi-major axis > 10000 AU. These comets are entering the inner Solar System for the first time, which gives us an excellent opportunity to study their composition. The Zwicky Transient Facility discovered Comet C/2020 V2 (ZTF) in November 2020. It was categorized as a dynamically new comet. In the talk, I will present the different results of this dynamically new comet C/2020 V2 (ZTF) using the photometric observations from TRAPPIST and spectroscopic observations from different observatories in India.

Gaining insight into radiative and variability phenomena of black hole X-ray binaries

Date: 2024-04-02
Speaker: Nazma Husain
Venue: online: https://meet.google.com/jqv-rbwg-unz

Abstract

Around 72 Black Hole X-ray Binary (BHXB) candidates have been discovered till now, showcasing variety of radiative phenomena and rapid variability in the X-ray lightcurve. This variability manifests itself as Quasi-Periodic Oscillations (QPOs) and broadband noise continuum in the power spectra. These properties offer a unique lens to study extreme gravitational environments in close regions to the black hole. Despite substantial research, our understanding of the origin and energy-dependent behaviour of the variability remains incomplete. I will focus on the modelling of variability, testing its different origins and exploring the spectral behaviour in different accretion regimes, using data from AstroSat, NICER, NuSTAR and Swift. In addition, I will discuss a peculiar case of a highly luminous BHXB, which exhibited non-standard disk behaviour. I will conclude by exploring future prospects to better our understanding of the accretion physics around different compact object systems.

Long-term X-ray temporal and spectral study of a Seyfert galaxy Mrk 6

Date: 2024-03-28
Speaker: Narendranath Layek
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Active galactic nuclei (AGNs) are the extremely luminous and most persistent energetic sources in the universe, powered by mass accretion onto the supermassive black hole (SMBH) at the centre of its host galaxy. The X-ray emission from AGN is vital to probe the physical processes in extreme gravity as it is thought to originate from a high-temperature electron cloud called the corona or Compton cloud, situated near the black hole. We present a long-term X-ray study of a nearby Active Galactic Nucleus Mrk 6, utilizing observations from XMM-Newton, Suzaku, Swift and NuSTAR observatories, spanning 22 years from 2001 to 2022. Mrk 6 is a relatively unexplored AGN that has exhibited “changing-look” behaviour in optical observations. However, its characteristics in the X-ray band have not been thoroughly investigated. We found a complex correlation between the soft (0.5 − 3.0 keV) and hard (3.0 − 10.0 keV) X-ray light curves. This result prompts a detailed investigation through spectral analysis, employing various phenomenological and physical models on the X-ray spectra. Based on the overall result obtained from X-ray spectroscopy, we found that, although Mrk 6 displays characteristics of a changing-look AGN (CLAGN) from optical observations, our X-ray spectral analysis did not show any significant variation in the X-ray luminosity and Eddington ratio over a period of 22 years. This indicates that in the X-ray regime, the source did not show any change in its behaviour during the observational period. In this talk, I will discuss our findings from temporal and spectral studies of Mrk 6, obtained from the analysis of X-ray data.

Towards the discovery and characterisation of Earth analogs with the PLATO mission

Date: 2024-03-21
Speaker: Dr. Alexandre SANTERNE
Venue: Google Meet: https://meet.google.com/wsj-jvrj-ipw

Abstract

PLATO is a mission from the European Space Agency that will be launch in 2026. It aims at discovering and characterising transiting exoplanets, down to Earth-size planets in the habitable zone of their host star (so called, the Earth analogs). Unlike the Kepler mission, the ground-based characterisation of the planet’s mass is fully part of the mission objective and ground-segment design. During this seminar, I will present the PLATO mission, including the most recent news about its development. I will also details what are the plan for the radial-velocity follow-up of the PLATO candidates and the discuss the current limitations.

A Multi-wavelength Study of Magnetic Cataclysmic Variables

Date: 2024-03-20
Speaker: Nikita Rawat
Venue: Google Meet: https://meet.google.com/nzs-gife-xpb

Abstract

Magnetic Cataclysmic Variables (MCVs) are a class of semi-detached binary star systems that consist of a white dwarf (WD) and a companion star, typically a Roche-lobe filling late-type main sequence star. The strong magnetic field associated with the WD distinguishes MCVs from other cataclysmic variables. It plays a crucial role in governing the accretion process in these binaries and also decides two distinct classes of MCVs: polars and intermediate polars (IPs). In polars, the magnetic field of the WD is strong enough (typically, in a range of 10-100 MG) to lock the whole system into synchronous (or almost synchronous) rotation. On the other hand, IPs have a weaker magnetic field (<10 MG) due to which they rotate asynchronously. The majority of IPs have the spin period of the WD, roughly one-tenth of the orbital period of the binary system. MCVs exhibit a wide range of timing (periodic and quasi-periodic) and spectral properties, more commonly in optical and X-ray wavelengths. These characteristics serve as valuable tools for unravelling and gaining insights into the intricate accretion processes within these systems. Among the MCVs, the appropriate classification of new sources is also essential to understand these systems thoroughly in terms of their accretion geometry. Further, the variable nature of accretion flow is one of the basic characteristics of an IP and has been observed in only a few IPs. In this talk, I will discuss the various photometric, spectroscopic, and polarimetric characteristics of a diverse sample of such objects in various evolutionary stages with spin-to-orbital period ratios ranging from 0.1 to 1.0 to gain insights into the various accretion flow scenarios these systems manifest.

Exploring the interplay of gravity, magnetic field, and turbulence at the hub of a Giant molecular cloud G148.24+00.41

Date: 2024-03-19
Speaker: Vineet Rawat
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

he relative importance of magnetic fields, turbulence, and gravity in the early phases of star formation is still not well understood. The plane of sky component of the magnetic field can be traced indirectly using the dust polarization of background starlight. In a recent work, we report the first high-resolution dust polarization observations at 850 μm around the most massive clump, located at the hub of the Giant Molecular Cloud G148.24+00.41, using SCUBA-2/POL-2 at the James Clerk Maxwell Telescope. In this talk, I will discuss the relative orientations of intensity gradients and local gravity with the magnetic field, which shows a dominant role of gravity in driving the gas collapse as the magnetic field orientations and gravity vectors seem to point towards the dense clumps. A better correlation of intensity gradients with the B-fields tells that matter is following the B-field lines or vice-versa. Our observation has resolved the massive clump into multiple substructures. We study the magnetic field properties of two regions, central clump (CC) and northeastern elongated structure (NES). Using the modified Davis–Chandrasekhar–Fermi method, we determine their magnetic field strengths to be around ∼24.0 ± 6.0 μG and 20.0 ± 5.0 μG, respectively. I will present the analysis done over CC and NES regions to understand their localized magnetic field-to-gravity ratio, magnetic criticality, Alfvénic state, and overall energy budget.

X-ray and Optical Studies of the Be/X-ray Binary IGR J06074+2205

Date: 2024-03-14
Speaker: Birendra Chhotaray
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

X-ray binaries are bright X-ray sources consisting of a compact object (white dwarf, neutron star, and black hole) and a main-sequence star as its companion. The compact object emits X-rays by accreting mass from the companion star, which is bright in optical. High-mass X-ray binaries (HMXBs) are one type in which the companion is a massive (M >10 M⊙, O or B spectral type) main-sequence star. In the Be/X-ray binary, a subtype of HMXB, a compact object (mostly neutron star) accretes mass from the circumstellar disc of its Be companion. We aim to understand the X-ray and optical properties of the Be/X-ray binary IGR J06074+2205. We have analyzed the X-ray data obtained from NuSTAR and NICER observatories to investigate the properties of the neutron star and optical data from MIRO and HCT to probe the properties of the Be companion. In this talk, I will discuss the time and luminosity-dependent properties of the Be/X-ray binary, which is obtained by timing and spectra analysis of X-ray and optical data.

Changing-State AGNs: Challenging our Understanding of AGNs

Date: 2024-03-13
Speaker: Dr. Arghajit
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Changing-state active galactic nuclei (CSAGNs) show the appearance and disappearance of broad emission lines in their UV/optical spectra on timescales of months to decades. The CSAGNs can not be explained by standard unified model AGNs that successfully explain all AGN phenomena in the last four decades. Here, we will discuss our current understanding of CSAGNs. We will also discuss open questions and future prospects.

Understanding the innermost geometry of accreting Seyfert galaxies using X-ray reverberation techniques

Date: 2024-03-11
Speaker: Dr. Mayukh Pahari
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Signs such as rapid X-ray fluctuations and significant variability in RMS values imply that the X-ray emitting source in an accreting black hole system is nearest to the black hole. However, details regarding its size, shape, and precise position remain unclear, often linked to the base of the radio jet along the black hole's vertical spin axis. X-ray reverberation, a vital observation technique involving coronal X-rays reflecting off the inner accretion disc, indicates a delayed, modified emission. Interestingly, the time delay measurements from such a modified emission in UV and optical follow standard thin disc theory; however, X-ray departs. In this talk, I will discuss how X-ray reverberation observationally differs from other wavelengths and how the lamppost geometric configurations can be used to explain the measured X-ray reverberations in Seyfert galaxies.

Hub-filament systems as progenitors of star cluster formation

Date: 2024-02-29
Speaker: Dr. M. S. Nanda Kumar
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

All stellar clusters form in hub-filament systems (HFS) and massive stars form in the hubs. The HFS structure is essentially that of what is generally referred to as molecular clouds (few to few tens of parsec) and hubs represent the central gravitating clump (1 pc) within HFS. I will first review our present knowledge of HFS and discuss how it can explain known properties of stellar clusters. Next, I will present our ongoing work to explore the star-gas relationship in the MonR2 HFS to understand whether molecular clouds are long lived resulting in slow star formation or vice versa. I will conclude by discussing the latest notions of how galaxies convert the interstellar medium (ISM) into stars and maintain the ISM life-cycle.

Heliospheric Propagation of Coronal Mass Ejections

Date: 2024-02-27
Speaker: Sandeep Kumar
Venue: Online:https://meet.google.com/uue-rpmm-upb

Abstract

Coronal Mass Ejections (CMEs) represent explosive releases of solar plasma and magnetic fields into the heliosphere, exerting significant influence on interplanetary space weather. The dynamics of CMEs involve the eruption of vast amounts of solar material and magnetic energy, driven by complex processes within the solar corona. As these charged particles are ejected into the heliosphere, they interact with the ambient medium. Upon arriving at the Earth they create disturbances that can affect the Earth's magnetosphere, ionosphere, and geomagnetic field. Understanding the initiation and propagation of CMEs is crucial for predicting their potential impact on space-based technologies, communication systems, and power grids. In this talk, I will discuss various factors affecting heliospheric propagations of coronal mass ejections and consequently how this interaction of the CMEs with the ambient medium can affect its impact on the Earth.

CARMENES: exoearths from Spain

Date: 2024-01-25
Speaker: Dr. José A. Caballero
Venue: Online: https://meet.google.com/fwp-cnit-rko

Abstract

Calar Alto high-resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs (CARMENES). In other words, it is a "machine for discovering planets like our Earth around the closest stars to our Sun". CARMENES took six years from a concept to the start of operations, and a couple more years of initial data collection until the first science publication, but now is revolutionising our knowledge on exoplanets and their stars in our immediate vicinity. In my talk I will describe what CARMENES is: (i) an ultra-stabilised two-channel spectrograph at an almost dedicated 3.5 m telescope in southern Spain that covers in high spectral resolution and without big gaps from 0.52 μm to 1.71 μm; (ii) a science project aimed at comprehensively searching for and studying planetary systems with nearby, bright, M-dwarf hosts, but that also investigates transiting planets around other stars; and (iii) the German-Spanish consortium that designed and built the instrument and that has operated it under guaranteed and legacy time observations.

Detecting tidal deformation and decay

Date: 2024-01-18
Speaker: Dr. Susana Barros
Venue: online : https://meet.google.com/myq-ynxh-zrk

Abstract

Tidal forces between short-period planets and their host stars are extreme. These lead to the deformation of the planet and the shrinkage of the planet’s orbit. Using the new ESA mission CHEOPS we are attempting to measure both these effects for a sample of exoplanets. Measuring the tidal deformation of the planet would allow us to estimate the second degree fluid Love number and gain insight into the planet's internal structure. Measuring the tidal decay timescale would allow us to estimate the stellar tidal quality factor, which is key to constraining stellar physics. WASP-103 was our first target since it had the largest estimated signature of the tidal deformation. I will present the first detection of the tidal deformation of a planet directly from its light curve. This allowed us to measure the Love number of WASP-103b. I will also present our measurements of the tidal decay of a few targets including WASP-103b and explore our future perspectives.

Cosmic Ray Transport in Magnetohydrodynamic Turbulence

Date: 2024-01-17
Speaker: Dr. Kiritkumar Makwana
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

Cosmic rays are affected by magnetic fields as they travel across the universe. Because of the turbulent and chaotic magnetic field lines in the intergalactic and interstellar medium and the heliosphere, cosmic rays are deflected and can exhibit random motion. It is essential, then, to understand how these cosmic rays diffuse out of a particular region of interest, carrying energy, mass, and momentum along with them. This diffusion process depends on the structure of the magnetic field lines in magnetohydrodynamic (MHD) turbulence. I will describe numerical simulations of this diffusion process using test particles in MHD turbulence simulation data. We investigate diffusion at both small and large scales (compared to the gyro-radius). At large scales we get normal diffusion whereas at small scales we observe Richardson diffusion, which is a type of super-diffusion. Different MHD wave modes are decomposed and diffusion due to them individually is also studied. The scaling of the diffusion coefficients with Alfven Mach number is calculated and roughly matches theoretical expectations. It is shown that the fast magnetosonic modes dominate in the cosmic ray diffusion process.

Probing the habitability conditions for the Earth-like exoplanets by their atmosphere characterization

Date: 2024-01-11
Speaker: Manika Singla
Venue: Online Mode: https://meet.google.com/meg-tqis-hzu

Abstract

In this talk, I will discuss about the ways to probe the habitability conditions on Earth-like exoplanets which lie in their host-star’s habitable zone as these exoplanets are the potential candidates for the habitability. We modelled the atmospheres of the Earth-like exoplanets by computing their reflection spectra, transmission spectra and the polarization phase curves. We also computed this for nine known Earth-like exoplanets (including Proxima Centauri b and Trappist-1e), which orbit around stars of spectral types G, K and M and estimated their maximum values of Bond Albedo. For this purpose, we assumed certain initial conditions on the atmospheric Temperature-Pressure profiles and the atmospheric chemical compositions and used the opacity data to solve the radiative transfer equations. I will further discuss about this in detail during the talk. Finally, I will discuss the profound effects of various surface compositions of the planet, clouds, orbital inclination angle, presence of biomolecules like H2O, CH4, O3, etc. on the reflection, transmission and the polarization of the planet. I will conclude the talk by discussing how our work will greatly enrich the upcoming search for the habitable exoplanets using big telescopes like HabEx, ARIEL, HWO, TMT, etc.

Probing the magnetic field and gas kinematics in the IRDC G11.11-0.12, Galactic "Snake"

Date: 2024-01-04
Speaker: Omkar Jadhav
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Numerous observational investigations have demonstrated that Infrared Dark Clouds (IRDCs) are important sites for studying the birth of embedded protostars including massive stars. IRDCs appear as absorption features against the galactic background in mid-infrared images. Magnetic fields are thought to play a crucial role in the mass accretion and stability of IRDCs. However, the exact role of magnetic fields in the early stages of star formation remains unclear. My present work focuses on the Galactic ‘Snake’ IRDC G11.11-0.12, which is known to host early phases of star formation activities. We have studied the plane of the sky magnetic field derived using the emission polarization and gas kinematics toward the entire IRDC. The existing JWST near-infrared images reveal signatures of star formation, which are new findings and has not been reported in existing literature. In this talk, I will present several new results about the Galactic "Snake" in more detail.

2023

Survey of Bare Active Galactic Nuclei in the Local Universe (z < 0.2): On the Origin of Soft Excess

Date: 2023-12-28
Speaker: Prantik Nandi
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Active Galactic Nuclei (AGNs), a subclass of Super-Massive Black Holes (SMBHs) emit radiation across the electromagnetic spectrum. The extreme luminosity of the AGNs arises from the accretion of matter onto the Super Massive Black Holes (SMBHs), forming an accretion disk that generates thermal photons. In the X-ray range, the inverse Compton scattering of these photons by the electron cloud, the Compton cloud, produces a power-law spectrum. Therefore, X-ray observations provide detailed insights into the inner accretion disk and Compton cloud in details. The soft excess, an excess emission below 2 keV, is an extraordinary feature in the X-ray spectra for most Seyfert 1 AGNs. The origin of soft excess is one of the major open questions in AGN research. Initially thought to be blackbody radiation, the high disk temperature corresponding to soft excess emission contradicts standard theories. some cases suggest that the reflection or Comptonization could be the origin of soft excess, But these theories are unable to explain all cases. These discoveries provoked the alternate origins of the soft excess. A recent study by Nandi et al. (2023) explores the origin of the soft excess for a sample of 21 Seyfert 1 AGNs with ‘bare’ nuclei. Long-term X-ray (0.5-10.0 keV) observations from observatories like XMM-Newton and Swift/XRT reveal a strong correlation between the luminosities of the primary continuum (3.0-10.0 keV) and soft excess (0.5-2.0 keV) across the sample. From this study, we conclude that the observed correlation between the soft excess and primary continuum luminosities could have originated due to inverse Compton scattering in the Compton cloud, and both the luminosities depend on the accretion rate only.

Study of short GRBs and their afterglows

Date: 2023-12-21
Speaker: Ashish Kumar Mandal
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

Gamma-ray bursts (GRBs) are brief intense flashes of gamma-rays produced at cosmological distances. They are classified as short or long based on the bimodality of the observed duration distribution of their prompt gamma-ray emission. This empirical classification was based on hypotheses that two classes have different progenitors. Long GRBs (LGRBs) have been firmly connected to the collapse of massive stars through the detection of associated Type Ibc core-collapse supernovae. But support for the connection between short GRBs (SGRBs) and merger of binary neutron stars (BNSs) or NS-BH binaries came from indirect observational evidence, population synthesis studies, and numerical simulations. The joint detection of gravitational wave GW170817 and a SGRB (GRB170817A) on 17th August 2017 from a BNS merger confirms that BNSs are progenitors of (at least some) SGRBs. This SGRB was 2 orders of magnitude nearer and 2 to 6 orders of magnitude less energetic than SGRBs with known distances. If it were at least 30% fainter, then most of our curent GRB detectors might have missed this event. In this seminar I will discuss on the properties of prompt gamma-ray emission, early afterglow in soft X-ray, UV, optical and late afterglow in IR, radio of SGRBs, some findings which support GRB classification criterion, and highlight sensitivity of few GRB detectors.

Multi-band Polarimetric Study towards the Cluster NGC 7380

Date: 2023-12-19
Speaker: Dr. Sadhana Singh
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The polarimetric study is an effective tool for providing information about the magnetic field orientation and will also reveal information about the properties of interstellar dust grains and their alignment. The polarization of background starlight through the interstellar medium is a result of the dichroic extinction of starlight through the aligned grains. In the seminar, I will present the multi-band polarimetric study of the open star cluster NGC 7380. Polarimetric observations are carried out in four filters using an ARIES imaging polarimeter from a 104-cm telescope of ARIES. The orientation of dust grains in the line of sight is found to be parallel with the Galactic magnetic field. Towards the east and southeast regions, the dust structure appears much denser than in other regions and is also reflected in the polarization distribution. Three groups of stars are identified in the Stokes plane, which suggests the presence of an additional weak magnetic field component. Additional results from the polarimetric analysis will be presented e.g. size distribution, polarizing efficiency, and foreground dust distribution. I will show the overall distribution of polarization and position angle towards Galactic anti-center direction and lastly, I will discuss future plans.

SPECULOOS: Hunting exoplanets of ultracool dwarfs with 1-meter ground-based telescopes network

Date: 2023-12-14
Speaker: Dr. Sebastián Zúñiga-Fernández
Venue: Online Mode : https://meet.google.com/vnm-saoz-izz

Abstract

The SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) project aims to perform a transit search on the nearest (< 40 pc) ultracool (<3000K) dwarf stars. The project is based on a network of 1m robotic telescopes, composed by the four ones of the SPECULOOS-Southern Observatory (SSO) in Cerro Paranal, Chile, one telescope of the SPECULOOS-Northern Observatory (SNO) in Tenerife, and the SAINTEx telescope in San Pedro Martir, Mexico. The prototype survey of the SPECULOOS project on the 60 cm TRAPPIST telescope (Chile) discovered the TRAPPIST-1 system, composed of seven temperate Earth-sized planets orbiting a nearby (12 pc) Jupiter-sized star. The project's main motivation is to discover potetially habitable planets well-suited for detailed atmospheric characterisation with upcoming giant telescopes, like the James Webb Space Telescope (JWST) and European Large Telescope (ELT). The SPECULOOS target list contains a homogeneous selected sample of close-by low-mass and ultracool stars.The targets have been selected as low-mass dwarfs starting from the Gaia DR2 catalogue, which has been cross-matched with the 2MASS point-source catalogue. Beside conducting observations of targets from the SPECULOOS input catalog, a fraction of the available observing time of the SPECULOOS network is used to carry out different science goals, the so-called annex programmes. A large annex programme is the support of space based transit search surveys such as K2 and TESS through the follow-up of transit candidates of late-type dwarfs.The upcoming Gaia DR3 release will largely improve the parallaxes and colours for the 40 pc sample, thus encompassing more targets that were excluded in our original target list. In these talks we are going to present our observation strategy, management of our archive and scientific products. We will discuss the first results of the survey and the synergy of our programs with the Transiting Exoplanet Survey Satellite (TESS) and JWST.

Astronomy in Africa for Achieving the Sustainable Development Goals

Date: 2023-12-13
Speaker: Dr. Mirjana Pović
Venue: Online Mode : Google Meet Link : https://meet.google.com/nec-gqmd-tuq

Abstract

Education and its contribution to science, technology and innovation are the key points to combat poverty in the long term. Education is also a key point to empower girls and women, which is fundamental to achieve the United Nations Sustainable Development Goals (SDGs). Astronomy is a powerful tool to promote education and science but, in addition, it is also one of the leading sciences to bring strong technological developments and innovation. Africa has an amazing potential due to its natural and human resources for scientific research in astronomy. The situation of astronomy and space science in Africa has changed significantly in recent years, becoming emerging fields across the continent, and never before has it been as possible to use astronomy for development as it is today. This talk will first summarize the current status of astronomy developments in Africa. Secondly, using as an example different activities carried out for the development of education, science and technology in Ethiopia, East Africa and across the continent, it will show how through these we can fight poverty in the long term and increase in the future our chances of achieving the UN Sustainable Development Goals (SDGs) for the benefit of our whole society.

Precision in Motion: Fabry-Perot Etalon as a Wavelength Calibrator for Extreme Precision Radial Velocity Methods

Date: 2023-11-30
Speaker: Shubhendra Nath Das
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

In this talk, I will explore how the Fabry-Perot etalon is changing the game in making radial velocity measurements super precise. I will start by understanding why it's crucial to calibrate wavelengths and look at the qualities a good calibrator should have. I'll show you how the Fabry-Perot etalon meets these standards, comparing it with other methods and pointing out its amazing benefits. At the end I will wrap up by discussing the stability and instrumental needs of the Fabry-Perot etalon to meet expectations.

Spectro-polarimetric Studies of Symbiotic Binaries

Date: 2023-11-23
Speaker: Arijit Maiti
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Symbiotic stars are binary systems consisting of a red giant and a hot radiation source, usually a white dwarf. The cool giant star loses material via Roche lobe overflow or through its stellar wind, which flows onto the hot compact star, usually via an accretion disk. Symbiotic binaries are of particular interest to astronomers as they can be used to learn about stellar evolution. They are also vital in the study of stellar wind, ionized nebulae, and accretion because of the unique interstellar dynamics present within the system. However, not a great deal is known about the structure and dynamics of such systems. Spectro-polarimetric studies of such systems represent an excellent technique for probing their 3D morphology and internal dynamics. Of particular interest are the spectro-polarimetric studies of the two broad Raman scattered lines at 6825 and 7082 angstroms seen in the spectra of most symbiotic stars. In this talk, I shall discuss the basics of symbiotic star systems, their properties, and how spectro-polarimetric techniques help in probing such systems with particular emphasis on the diagnostic potential of the Raman scattering process. I shall conclude the talk by discussing how ProtoPol (a prototype spectro-polarimeter currently under development in PRL) with a medium resolution of (R ~ 7000) can help conduct such spectro-polarimetric studies.

Evolution of X-ray Instrumentation and Astronomy

Date: 2023-11-10
Speaker: Neeraj Tiwari
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

X-ray astronomy, a latecomer in the field of observational astronomy, remained unexplored until the 1960s due to the inherent challenge of cosmic X-rays being unable to penetrate Earth's atmosphere. Since its inception, X-ray astronomy has undergone a remarkable evolution, reshaping our understanding of the cosmos. It began its journey with experimental X-ray observations using sounding rockets, then progressed to balloon-borne missions, and eventually to the deployment of advanced satellite-based observatories. This evolutionary path has illuminated the field of astronomy, revealing complex high-energy phenomena associated with black holes, neutron stars, and active galactic nuclei. X-ray astronomy has played a vital role in understanding the mysteries of accretion processes, supernovae, X-ray binaries, and gamma-ray bursts. It has also provided crucial insights into the formation and evolution of galaxies, the behavior of matter under extreme conditions, and the dynamics of celestial collisions. The pursuit of precision and clarity in X-ray observations was made possible through the evolution of technology over the last six decades, which led to the development of groundbreaking X-ray telescopes like Chandra, XMM-Newton, and NuStar. These missions owe their success and improved performance to the advancements in technology, enabling us to address long-standing questions within the field of astronomy. In this seminar, we will journey through the evolution of X-ray astronomy, exploring its inspiring history and the ways it has deepened our understanding of high-energy astrophysical processes. We will also explore the ongoing challenges in the development of the next generation of X-ray telescopes and the promising opportunities they hold for the future of this captivating branch of astronomy.

VLT/HiRISE: Direct characterization of young giant exoplanets at high spectral resolution

Date: 2023-11-09
Speaker: Dr. Arthur Vigan
Venue: Online link : https://meet.google.com/mhu-ukug-gyy

Abstract

A major endeavor of this decade is the direct characterization of young giant exoplanets at high spectral resolution to determine the composition of their atmosphere and infer their formation processes and evolution. Such a goal represents a major challenge owing to their small angular separation and luminosity contrast with respect to their parent stars. Instead of designing and implementing completely new facilities, it has been proposed to leverage the capabilities of existing instruments that offer either high contrast imaging or high dispersion spectroscopy, by coupling them using optical fibers. In this seminar I present the implementation and first on-sky results of the HiRISE instrument at the very large telescope (VLT), which combines the exoplanet imager SPHERE with the recently upgraded high resolution spectrograph CRIRES using single-mode fibers. The goal of HiRISE is to enable the characterization of known companions in the H band, at a spectral resolution of the order of R = 100 000, in a few hours of observing time. I will present the science case, the main design choices and the technical implementation of the system, and the first performance assessment after commissioning.

X-Ray Spectral Properties of Narrow Line Seyfert 1 galaxy

Date: 2023-11-02
Speaker: Ms. Isha Mahuvakar
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

Narrow-line Seyfert 1 (NLS1) galaxies are a subclass of Active Galactic Nuclei (AGN), showing relatively narrow width (FWHM < 2000 km s-1) of broad permitted emission lines in their optical spectra. These galaxies are supposedly powered with low mass supermassive black holes (105 – 107 Msun) and exhibit high accretion rates. In this seminar I shall present X-ray spectral properties of a NLS1 galaxy B3 1702+457 using multi-epoch XMM-Newton and AstroSAT observations. Our preliminary results show that the 0.3-10 keV X-ray spectra of this NLS1 can be fitted with an absorbed power law plus reflection component and Fe Ka emission line. We also find the evidence for absorption edges at soft energies. The multi-epoch X-ray observations hints for the flux as well as spectral variability. In this talk, I shall emphasize on the peculiarity of NLS1 X-ray spectral properties.

Multiwavelength study of hot and exotic stellar populations in star clusters

Date: 2023-10-23
Speaker: Ms. Sharmila Rani
Venue: Online

Abstract

Star clusters are dynamically active stellar systems where interactions and collisions, especially involving binaries, are quite frequent and result in stellar exotica such as blue straggler stars (BSSs), cataclysmic variables, etc. Globular clusters (GCs), being old and dense, host low-mass stars in various stages of evolution, making them ideal for investigating the end stages of low-mass star evolution and the origins of exotic stellar populations, which are not yet fully understood. Ultraviolet (UV) studies conducted so far using HST and GALEX data have revealed the presence of intriguing stars, such as extreme horizontal branch (EHB) and blue hook (BHk) stars in GCs, making them enigmatic systems. Furthermore, the discovery of multiple stellar populations (MSPs) in both types of clusters through UV observations has provided new perspectives on their formation and evolution. We examined the properties of late-stage, UV-bright and exotic stellar populations in targeted star clusters using the multiwavelength data from the ground as well as space-based observatories. We primarily have employed the UV data from the Ultraviolet Imaging Telescope (UVIT)onboard AstroSat, India’s first multi-wavelength observatory, in both far-UV (FUV) and near-UV (NUV) bands. In this talk, I will present an in-depth analysis of four clusters, consisting of two intermediate-mass GCs (NGC1261 and NGC2298) and two open clusters (OCs) (NGC188 and NGC2818). We constructed the UV-optical colour-magnitude diagrams (CMDs) for member stars and compared them with theoretical isochrones to probe the multiple population phenomenon in GCs. Furthermore, based on the atmospheric parameters estimated using the SED fitting technique, we constrained the formation scenarios of the hot horizontal branch (HB) and exotic stars in clusters. Additionally, FUV imaging analysis of a Planetary Nebula (PN) within the field of OC NGC2818 strongly suggests its membership in the cluster, offering a unique opportunity to test stellar evolution models.

On the nature of AGN in dust-obscured galaxies

Date: 2023-10-12
Speaker: Abhijit Kayal
Venue: Thaltej Seminar room (113)

Abstract

We explore the nature of Active Galactic Nuclei (AGN) hosted in high-redshift (0.59 <= z <= 4.65) dust-obscured galaxies (DOGs) by performing X-ray spectral study of DOGs detected in the XMM-SERVS deep field. The X-ray spectra fitted with a simple absorbed power law plus reflection component or with a torus-based reflection model reveal that a substantial fraction (30 per cent) of our sample DOGs are heavily obscured (N_H > 1023 cm-2) AGN. The absorption-corrected 2.0 - 10 keV X-ray luminosity suggests them to be luminous quasars. The NH versus Eddington ratio diagnostic plot infers that our DOGs host a heterogeneous population of AGN containing heavily obscured AGN as well as reddened quasars. We find that only a few of our sources are likely to belong to an early phase during which accretion and obscuration peak, while the remaining sources possibly belong to a late phase during which radiative feedback from dominant AGN blows away obscuring material. Since X-ray detection is limited only to bright AGN, we also use deep multi-frequency radio continuum surveys, which help us to unveil a large fraction of low-luminosity AGN hosted in DOGs. In this talk, I shall emphasize the importance of deep radio surveys to uncover the obscured population of AGN.

Accretion Disk-Corona Connection in Active Galactic Nuclei

Date: 2023-10-05
Speaker: Dr. Indrani Pal
Venue: Online

Abstract

One of the important components of active galactic nuclei (AGN) is the corona, believed to be responsible for the luminous X-ray continuum that is observed in the radio-quiet category AGN. The hot electrons within the corona Compton up scatters the optical and UV photon from the accretion disk to X-ray energies. In spite of the corona being considered an integral part of AGN, much is still unknown about its nature, geometry, location and physical properties. Using data taken from X-ray telescopes such as the NuSTAR and XMM-Newton, a systematic investigation was carried out on the nature of the corona using X-ray spectral techniques. On analysis of a sample of about 100 Seyfert type AGNs, the coronal temperature could be constrained for about 75 AGN. Of these, variation in the temperature of the corona was found in two. Though spectral fits could provide us with important characteristics of the corona, a key unknown is the geometry, which could be constrained using X-ray polarization observations. Using X-ray polarization data from the Imaging X-ray Polarimetry Explorer (IXPE), launched by NASA in December 2021, we could constrain the geometry of the corona in IC 4329A, one among the four radio-quiet AGN for which IXPE observations are available as of today. Details of the results on the coronal properties of AGN obtained from our systematic investigation, as well as plans for the future will be presented.

Chandrayaan-2 XSM: Bits and Bytes to Science Data Archive

Date: 2023-10-04
Speaker: Mithun N. P. S
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

Data acquired by various astrophysical observatories are generally made available to the scientific community worldwide often after a short proprietary lock-in period for the instrument team or the proposers of the observations. Such open data policies have played an important role in advancement in our understanding of various astrophysical processes through archival data analysis. While the data policies of ground based observatories differ, almost all space based observatories, which are often fully funded by public money, are required to make the data public with varying lock-in periods. However, the usability of the archival data crucially depends on how well the data archive is prepared and documented. It is also essential to provide the users with analysis tools that apply required corrections and calibration to the observational data which in most cases are quite involved. The Solar X-ray Monitor (XSM) of the Chandrayaan-2 mission has been observing the Sun from lunar orbit and providing measurements of soft X-ray spectra of the Sun for just over four years now. The raw data acquired gets downloaded at designated passes over Indian Deep Space Network (IDSN) at Bylalu, Bangalore. The data then undergoes initial processing at Indian Space Science Data Centre (ISSDC) after which it is made available to the Payload Operations Center (POC) at PRL. Higher levels of data processing and validation are carried out at PRL and the final data archive is hosted at ISSDC PRADAN portal. PRL also maintains an interactive website for users to examine the XSM light curves. This entire chain from data reception to data archival and updating of the website is automated with a suite of software tools that we call as XSMDMS: XSM Data Management System, which in turn uses the XSM Data Analysis Software. In this talk, I will discuss how the XSM data makes its way from bits and bytes to the science data archive.

Introduction to Gamma Ray Bursts (GRBs)

Date: 2023-09-14
Speaker: Ashish Kumar Mandal
Venue: Seminar Room # 113/114 (Thaltej campus)

Abstract

Gamma Ray Bursts (GRBs) are short, intense flashes of gamma rays coming from deep space. They were first discovered (in 1967) by VELA military satellites of the USA launched to govern the 1963 nuclear test-ban treaty. They are observed across the electromagnetic spectrum, from radio to very-high-energy gamma rays. At present, space-based observatories detect, on average, approximately one GRB per day. The extensive observational and theoretical efforts have confirmed them as highly relativistic explosions, confirmed their distance scale, pinpointed their host galaxies, identified the systems that create them and opened them as astrophysical probes throughout the Universe. GRBs are now a major astrophysical tool to probe the final stages of stellar evolution and the creation of supernovae, to examine the creation and propagation of relativistic jets and their impact on the Universe at large, and to use as indicators to identify galaxies across cosmic time-some in the earliest epochs of Universe. The extreme nature of both their progenitors and emission mechanisms makes them the most promising sources for joint electromagnetic and gravitational wave detection. This is demonstrated by the recent (2017) detection of gravitational waves (GW170817) and short GRB (GW170817A) from a merger of the double neutron-star binary system. In this seminar, I will talk about the brief history of GRBs, their observations and properties, different dedicated GRB missions, detection methods, understanding so far, etc., and finally, I will connect you to my work.

Intra-night optical variability of radio-quiet narrow-line Seyfert-1 galaxies

Date: 2023-09-04
Speaker: Dr. Vineet Ojha
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Variability studies of Active Galactic Nuclei (AGNs) play a powerful diagnostic tool in understanding the physical processes occurring in objects that are unresolved by direct imaging with currently available techniques. Here, we report the first attempt to systematically characterize intra-night optical variability (INOV) for a sample of seven radio-quiet and/or radio-silent narrow-line Seyfert 1 galaxies (RQNLSy1s) that had shown recurring flaring at 37 GHz in the radio observations at Metsähovi Radio Observatory (MRO), indicating the presence of relativistic jets in them, but no hints of jets in the recent radio observations of Karl G. Jansky Very Large Array (JVLA) at 1.6, 5.2, and 9.0 GHz. A total of 28 intra-night sessions, each lasting > 3 h, was conducted with 1-2.5m class telescopes of ARIES and PRL for the INOV study of this sample. The resulting level of INOV from this sample is found to be statistically comparable to those observed in γ-ray-detected radio-loud NLSy1s (MBH ∼ 10^7 M⊙ ), that display blazar-like INOV. Thus, it appears that even lower-mass (MBH ∼ 10^6 M⊙ ) RQNLSy1 galaxies can maintain blazar-like activities. Furthermore, based on the optical behavior of these seven RQNLSy1s, we infer that INOV from the current sample of RQNLSy1s could be either due to a twisted in-homogeneous jet or magnetic re-connection in the magnetosphere of the black hole. In this talk, I will also discuss the optical study of the most distant gamma-ray-detected NLSy1 (z = 1.344), whose observations were taken from the PRL 2.5m telescope.

Comet Observations from the 3.6m Devasthal Optical Telescope (DOT)

Date: 2023-08-31
Speaker: Goldy Ahuja
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

This talk presents the results from the first observations of comets using the ADFOSC (ARIES Devasthal Faint Object Spectrograph and Camera) on the 3.6m Devasthal Optical Telescope (DOT). The famous green comet C/2022 E3 was observed post-perihelion on 16 Feb and 22 Feb 2023. The talk will focus on the challenges faced during the reduction and calibration of the data from the ADFOSC spectrograph. The procedures for reducing the data will be discussed, and the production rate ratio will be presented and compared with results from the TRAPPIST group.

Development of Spectro-polarimeters for PRL Telescopes

Date: 2023-08-28
Speaker: Arijit Maiti
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Polarization measurements in astronomy is one of the most challenging observational techniques given its photon-hungry nature. While imaging polarimetry in broadband filters have been a very successful technique to explore a variety of astro-physical situations, Spectro-polarimetry is rather rare, and not so many instruments available to explore this domain. Spectro-polarimetry is the study of polarization profiles of various features in the spectrum of any astro-physical object. Measuring the wavelength dependence of the polarization of radiation from such sources can reveal valuable and interesting constraints on the nature of objects observed. Here at PRL, we are in the process of developing two such instruments having spectro-polarimetric capabilities. Mt. Abu Faint Object Spectrograph and Camera–Echelle Polarimeter (M-FOSC-EP) is a 2-channel multimode instrument equipped with a low-resolution spectroscopic and imaging arm and an intermediate-resolution (R~15000) spectro-polarimetric arm. As a precursor to this instrument, a prototype spectro-polarimeter called ProtoPol has also been designed, using completely off-the-shelf optical components, for medium-resolution (R~6000) spectro-polarimetric studies. In this seminar, I will talk about the basics of the technique of spectro-polarimetry and a few science cases that could be benefitted from it. I will talk about the design and specifications of both instruments and share the latest assembly status of ProtoPol which currently is in the development stage. I will also discuss a simulation code that is being developed to simulate the performance of the M-FOSC-EP instrument.

Chemical Analysis of Nearby M Dwarfs Based on High-resolution Near-infrared Spectra Obtained by the Subaru/IRD Survey

Date: 2023-08-24
Speaker: Dr. Hiroyuki Tako ISHIKAWA
Venue: Online

Abstract

Chemical analyses of M dwarfs are scarce but necessary to explore the formation scenario and internal structure of planets orbiting them. I will talk about our spectral analyses of 71 mid- to late M dwarfs (2800 < Teff < 3400 K) observed in the Subaru/IRD planet search project (IRD-SSP survey). We used the high-resolution (∼70,000) near-infrared (970–1750 nm) spectra to measure the effective temperatures and the abundances of Na and Fe by the line-by-line analysis based on model atmospheres. For 13 objects of the targets, we also measured elemental abundances of Mg, Si, K, Ca, Ti, V, Cr, Mn, and Sr. The radial velocities we measured from the spectra were combine with Gaia astrometry to calculate the Galactocentric space velocities UVW. While the [X/Fe] distributions are comparable to those of nearby FGK stars, most of which belong to the thin-disk population and the most metal-poor object, Barnard's Star, could be a thick-disk star. The UVW velocities also support this. The results raise the prospect that near-infrared spectra of M dwarfs obtained in the planet search projects can be used to grasp the trend of elemental abundances and the Galactic stellar population of nearby M dwarfs.

Spectral Ages of Remnant Radio Galaxies

Date: 2023-08-17
Speaker: Dr. Sushant Dutta
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The cessation of active galactic nucleus (AGN) jet activity can be well detected in radio galaxies with characteristic features such as absent core and jets, amorphous lobes of low-surface-brightness and strong spectral curvature. Radio galaxies with no AGN-jet activity are known as remnant radio galaxies (RRGs) which are believed to be rare objects. The scarcity of RRGs can be explained with the fact that they can be observed only over a relatively short time-window before they fade away due to radiative and dynamical energy losses. The time-scales of the active phase, remnant phase and AGN duty cycle are vital to understand the evolution of radio galaxies and related feedback. I shall present our ongoing work on estimating spectral ages of RRGs by modelling their radio spectral distributions (SEDs) with physically motivated models, namely, continuous injection (CION) and continuous injection-off (CIOFF) models. For our RRGs, we have obtained densely sampled SEDs across 144 MHz - 1.5 GHz using multi-frequency radio observations from LOFAR, uGMRT, MeerKAT and VLA. In this talk, I shall emphasize on the importance of newly acquired radio observations at band-3 (300-500 MHz) and band-4 (550-850 MHz) from uGMRT and L-band from MeerKAT, carried out under the superMIGHTEE and MIGHTEE projects respectively, which help us in placing better constraints on the spectral age estimates.

DWARF NOVAE: ACCRETION POWERED COSMIC FIREWORKS

Date: 2023-08-14
Speaker: Akash Sundriyal
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

A dwarf nova outburst is a very special and characteristic event undergone by a cataclysmic variable. These events are associated with the accretion disc as confirmed by recent observations. During a dwarf nova episode, the accretion disc brightens up by 2-5 magnitudes in typically a few days. This is a result of enhanced accretion onto the white dwarf as the disc becomes unstable. These events typically last for 2-20 days and occur semi-regularly. In this talk, I will briefly cover dwarf nova phenomenon and the mechanism responsible for it. The effect of mass-transfer rate and its role in shaping the outbursts will also be discussed. I shall also discuss the SED and TESS light curve analysis that has been carried out for the long-orbital period system V1948 Cyg.

Automatic Learning for the Rapid Classification of Events: ALeRCE

Date: 2023-08-03
Speaker: Amelia Bayo
Venue: Online

Abstract

Arguably, time domain (probably together with multi-messenger astronomy) is bound to change both, the way we "do observational astronomy" and what we know (and do not know) about the universe. Infrastructure like the Vera Rubin Observatory (VRO), for instance, will soon provide us with a much more dynamic view of a large portion of the night sky. The degree to which that kind of view can be scientifically exploited will heavily depend on our ability to generate more efficient data-flows and analysis tools, and the access to the right mid-size infrastructure for follow up. The Zwicky Transient Facility (ZTF) has been and is being used to test, improve, and prepare ourselves to deal with large data streams. All seven community brokers that will provide access for the general community to interesting variable objects detected by VRO are taking ZTF as the "training ground". In this talk I will present the work done by ALeRCE in particular (the only southern hemisphere broker) and how their services can provide a very interesting curated set of sources to be followed up by facilities related to your institute.

Ionic emissions in Comet C/2020 F3 (NEOWISE)

Date: 2023-07-31
Speaker: Dr. Aravind K
Venue: Room No. 113/114, Thaltej Campus, PRL

Abstract

The composition of molecular Nitrogen relative to CO and H2O has been sparsely studied in comets. The abundance of these neutral gases in the coma can be probed using optical spectroscopy only via their ionic emissions. While detecting and analysing them is crucial, it is equally challenging due to their very low density. Even though the Great Comet of 2020, C/2020 F3 (NEOWISE), has been studied in detail via very high-resolution spectroscopy, the presence of ionic emissions has not been reported in it. Long slit low-resolution spectroscopy of such bright comets can help analyse the spatial profile in great detail. This seminar will discuss the detection of the ionic emissions N2+, CO+, CH+ and H2O+, along with a few unidentified and unusual emissions in the spectacular comet C/2020 F3 (NEOWISE). While the computed relative abundances and their implications will be discussed, we also try to compare the unusual emissions observed in comet C/2020 F3 with the emissions which were also seen in the spectrum of comet C/1987 P1 (Bradfield) but remained unnoticed at the time.

Shock induced dust formation in novae

Date: 2023-07-27
Speaker: Dr. Ruchi Pandey
Venue: Room No. 113/114, Thaltej Campus, PRL

Abstract

Novae are the only objects in which it has been possible to observe directly all aspects of circumstellar dust formation on a frequent basis. Novae dust forms within a short time frame of typically 30 to 100 days after an outburst, allowing them to serve as test beds for understanding the formation and evolution of astrophysical dust. However, dust formation in the hostile environment of novae ejecta has been an open question for many decades. Several attempts have been made to understand the physical and chemical conditions required to dust formation in novae ejecta and its relation with the observable parameters. To explain the mechanisms underlying dust formation, numerous hypotheses have been proposed. In recent years, an intriguing hypothesis involving shock-induced formation of dust in novae has been proposed. A recent study of Nova V2891 Cyg by a PRL team provides, most likely, the first observational evidence of such a method of dust formation in novae ejecta. This seminar will commence with a concise introduction to the phenomenon of dust formation in novae, highlighting its significance and the current gaps in knowledge. Further, we will present the phenomenological cloudy modelling of Nova V2891 Cyg in order to illustrate the origin of some observational signatures of shock-induced dust formation.

Unravelling the Origin Mystery of anomalously large lithium in red giants

Date: 2023-07-20
Speaker: Prof. Eswar Reddy
Venue: Room No. 113/114, Thaltej Campus

Abstract

In general, lithium gets destroyed in stars as they evolve. However, a small group of stars show excess lithium and, in some cases, more than the abundance seen in their natal clouds. The problem remained a puzzle for more than 40 years. With the availability of high-quality spectra for a large number of stars and asteroseismic data, we could now locate the source of high lithium abundance in red giants. In this talk, I will discuss and provide the current understanding of the origin of high Li in stars.

The Giant Molecular Cloud G148.24+00.41: Gas Properties, Kinematics, and Cluster Formation at the Nexus of Filamentary Flows

Date: 2023-07-20
Speaker: Mr. Vineet Rawat
Venue: Room No. 113/114, Thaltej Campus

Abstract

Giant molecular clouds (GMCs) are the cradles of young star clusters in which most stars form. Massive to intermediate-mass clusters play an important role in the evolution and chemical enrichment of the Galaxy through radiation and winds. However, the formation mechanism of stellar clusters, in particular, intermediate to massive clusters, is still not clear and a topic of debate. Determining and evaluating the physical conditions, kinematics, structures, and dynamics of massive molecular clouds (>10^5 Msun;) at their initial stages of evolution, such that stellar feedback effects are not significant there, are key to understanding the formation of stellar clusters. In this talk, I will present a case study of one such GMC, G148.24+00.41 having mass ~ 10^5 Msun; and size ~20 pc, based on CO molecular line data to understand its cluster formation scenario. I will present the role of filamentary flows in assembling the gas at the central gravitational potential of the cloud. I will discuss about the massive cluster formation scenarios given in the literature and their possibility in the G148.24+00.41 cloud.

Implications from Galactic Archaeology to Exoplanets

Date: 2023-07-13
Speaker: Diogo Souto
Venue: Online

Abstract

M-dwarf stars, comprising approximately 70% of all stars in the Milky Way, are the most abundant type of star. However, they remain relatively understudied when it comes to their chemical abundances. This knowledge gap primarily arises from the complexity of their optical spectra, which are heavily influenced by strong molecular bands like TiO and VO. By leveraging near-infrared high-resolution spectra, we have been able to precisely determine stellar parameters and the abundances of up to fourteen elements in these stars. Given the increasing number of Earth-sized exoplanets discovered orbiting M dwarfs, there is a growing interest in characterizing these stellar objects. In this talk, we will explore how studying stellar abundances in M dwarfs can significantly enhance our understanding of Galactic archaeology to exoplanets.

Design, Development and Performance Modelling of WALOP Polarimeters for PASIPHAE Survey

Date: 2023-03-16
Speaker: Dr. Siddharth Maharana
Venue: Thaltej Seminar room (113)

Abstract

PASIPHAE survey aims to create the first large sky magnetic field and dust cloud tomographic map of the Galactic polar regions using stellar polarimetry and GAIA stellar distances. Two WALOP (Wide-Area Linear Optical Polarimeter) instruments, to be mounted on 1 m class telescopes in South Africa and Greece are currently under development to work as survey instruments for the PASIPHAE program for creating the stellar polarization catalogue. Scheduled for commissioning in 2023, the WALOPs are being designed to operate with the combined capabilities of one-shot four camera linear polarimetry, low polarization systematic (polarimetric accuracy of 0.1%) and a large field of view of 30×30 arcminutes, which in combination will make these unique astronomical instruments. Operating in the SDSS-r broadband and narrowband filters between 500-700 nm, for each exposure, four images of the full field corresponding to polarization angles of 0, 45, 90 and 135 deg will be generated and carrying out differential photometry on these images will yield the linear Stokes parameters. We have developed a complete design and calibration strategy for WALOPs to achieve its technical goals. In this talk, I will present an overview of the instrument design, development, performance modelling and calibration routine for the instruments and their current status.

Near infrared background with the 1.2 m telescope at Mount Abu

Date: 2023-02-23
Speaker: Ms. Prachi V. Prajapati (Scientist/Engineer SD, A&A division PRL)
Venue: Room no. 113/114 Thaltej

Abstract

Study of the background in astronomical observations plays a crucial role in understanding the characteristics of background noise. In this seminar, I will be discussing the estimation and analysis of the Near Infrared (NIR) background for the astronomical site at Mount Abu. Data obtained from the NIR observations of various sources using the Near Infrared Camera/Spectrograph (NICS) on the 1.2 m telescope are used in this study to derive the NIR background estimates. We notice seasonal variations in these values, with the summer months showing slightly brighter background compared to the winter. A comparison of synthetic atmospheric transmission at Mount Abu with other Indian observatories at various altitudes will also be presented. We identify the plausible contributors to the NIR background in our observations and discuss the potential ones that can contribute to the noticed seasonal variations. Overall, we find that the NIR background does not show any systematic variation over the period of the observations from 2010 to 2019.

Accretion around black holes: The geometry and spectra

Date: 2023-02-09
Speaker: Narendranath Layek
Venue: Room no. 113/114 Thaltej

Abstract

The most energetic phenomena of our universe are generally powered by the process of accretion onto compact objects. This process releases energy through radiation. When accreting material has angular momentum, it forms a disc-like structure around the central object at the equatorial plane is called an accretion disc. There are various types of disc mechanisms present in the literature. Between them, the Standard disc, proposed by Shakura and Sunyaev in 1973, is the most preferred disc model, which is used widely to explain the disc accretion around any compact object. This model predicts disc properties like spectral energy distribution and luminosity and has been validated by observations of various astrophysical systems. In this talk, I'll give a brief overview of various accretion processes, including disc accretion and the associated radiation mechanisms.

From hot Jupiters to super-Earths: Exoplanets and beyond

Date: 2023-01-31
Speaker: Dr. Priyanka Chaturvedi
Venue: Thaltej Seminar room (113)

Abstract

The search for planets beyond our solar system is one of the biggest scientific quests at present. The first exoplanet detection by the radial velocity (RV) method nearly three decades back was awarded the Nobel prize in Physics. Since then, the number of exoplanets found has increased exponentially with nearly 5000 exoplanet detections till date. We have learnt that diverse planetary systems form with orbits, densities, and planetary atmospheres that are very different from the planets in our solar system. In my talk, I will discuss planet demographics across the stellar spectral range focusing on large planets around the solar-type stars and smaller planets around the late spectral type stars. Despite the advent of large telescopes and precise spectrographs, the detection of smaller planets is largely limited by the intrinsic stellar jitter. I will talk about the ways that we devise to disentangle this stellar jitter from the planet signature. I will finally highlight the role of small to medium-size telescopes for exoplanet characterization in the era of current and future space missions like TESS and PLATO.

On the origin of a kiloparsec size superbubble in the JWST images of the "phantom galaxy" NGC628

Date: 2023-01-23
Speaker: Prof. Yalia Divakara Mayya
Venue: Thaltej Seminar room (113)

Abstract

NGC628 is the first nearby galaxy for which JWST data became available to the public. The most striking characteristic of the publicly released MIRI image is the presence of a large number of "holes" in an otherwise bright mid-infrared (MIR) emitting disk. This porous structure has given it a popular nickname "phantom galaxy". The "holes" are most often expanding bubbles or superbubbles that are created by the mechanical power output by the massive stars in young star clusters. However, few cases exist where the stellar population that was responsible for the creation of the bubble was unambiguously identified. We here analyze the largest of the bubbles in the JWST/MIRI image of NGC628, measuring 1.3 kpc in diameter to understand the origin of such large bubbles. We combined the JWST NIRCam and MIRI dataset with archival images from the HST, ALMA, VLA and MUSE, to identify the resolved population that might be responsible for the creation of the bubble, and to map the multiphase morphology and kinematics of the gas in the shell surrounding the bubble. The bubble is dominated by the molecular gas and is expanding at 12 km/s velocity. We find conclusive evidence for the presence of a resolved stellar population of ages between 5 to 50 Myr inside the bubble, whose collective mechanical power output is sufficient to explain the presently observed radius, velocity and the shell mass. In the talk, I will discuss the star formation scenario inside the bubble and the conditions that favour the formation of large superbubbles.

Hard X-ray polarimetry with CZT Imager onboard AstroSat: An overview of the technique and recent scientific findings

Date: 2023-01-12
Speaker: Dr. Tanmoy Chattopadhyay
Venue: Thaltej Seminar room (113)

Abstract

CZT Imager onboard AstroSat is primarily a hard X-ray spectroscopic instrument but provides sensitive polarization measurements above 100 keV for bright X-ray sources, e.g. Crab pulsar and the Cygnus X-1 black hole system. With an accumulated 800 ks of observation, CZTI provided till date statistically the most significant polarization measurement for Crab. Phase-resolved polarimetry of the pulsed radiation of Crab was also attempted for the first time in X-rays and the findings provided interesting insights to the emission site of hard X-rays. In the case of Cygnus X-1, the recently dedicated long AstroSat observations show a strong dependence of polarization properties on the spectral state of the source. This is an interesting finding and might be useful in understanding possible disk-jet interplay in black hole systems. Another important characteristic of CZTI is the increasing transparency of the support structure in hard X-rays, which enables it to function as a wide field hard X-ray monitor providing an unique opportunity for GRB detection and polarization measurement. From a sample of a large number of GRBs detected by CZTI in the last 5 years, we attempted polarization measurement for 20 bright GRBs. Time integrated analysis of the prompt emission yields low or no polarization indicating that GRB prompt emission is highly structured leading to possible changes in the polarization angle within a burst which has also been seen independently for some of the bright AstroSat GRBs. A large sample of such polarization measurements in the future is supposed to shed more light into the emission mechanism of GRB prompt emission. In this presentation, I would briefly discuss these recent scientific findings from CZTI. Polarization measurement in hard X-rays for CZT based large area instruments is challenging because of multiple factors, e.g. large background, difficulty in reconstructing the Compton events properly in a single crystal detector plane, systematic effects because of square pixel geometry and the effect of charge sharing. Polarization measurements also critically depend on the accuracy of the mass model of the instrument and the spacecraft particularly for off-axis sources like GRBs. We inspected each of these effects carefully and developed our analysis algorithm over the last few years. I would highlight these challenges and their effects in the context of CZTI polarimetry technique.

Development and characterization of fast low noise X-ray detectors for the next generation astronomical observatories.

Date: 2023-01-10
Speaker: Dr. Tanmoy Chattopadhyay
Venue: Thaltej Lecture room (113)

Abstract

The next generation of large X-ray astronomical missions (e.g. the Lynx Great Observatory and AXIS probe class mission concepts) are planned to have an order-of-magnitude larger collecting areas combined with exquisite mirror quality, enabling unprecedented exploration of the faint and high redshift X-ray universe. An essential component of these mission concepts is the development of focal plane instrumentation with fast readout and excellent noise performance - crucial especially for the soft X-ray performance - on the order of single electrons. MIT Lincoln Laboratory (MIT-LL), MIT Kavli Institute (MKI) and Stanford University (SU) have together made substantial improvements in developing next generation X-ray CCDs, and associated readout electronics to support these detectors. We recently characterized a prototype CCD at 4 Megapixel/s (40 times faster than Chandra CCDs) with good noise performance. In parallel, we are also working on a novel Single electron Sensitive readout stage (SiSeRO) for CCDs, and potentially active pixel sensors, which can in principle provide even greater responsivity and better noise performance than contemporary JFET technology. The SiSeRO technology uses a p-MOSFET transistor with a depleted back gate region beneath the transistor channel. The transistor source-drain current is modulated by the transfer of charge into the back gate region. The first SiSeRO prototype devices already achieve a charge to current conversion gain of >700 pA per electron, an equivalent noise charge (ENC) of 4.5 electrons root mean square (RMS), and a full width half maximum (FWHM) of 130 eV at 5.9 keV at a readout speed of 625 Kpixel/s. Importantly, utilizing the benefit that the charge signal remains unaffected by the readout process in these devices, we have also been able to implement Repetitive Non-Destructive Readout (RNDR) on our prototype devices, achieving significantly improved ENC performance. In this talk, I will discuss the working and characterization test results for these new fast, low noise X-ray CCDs and the novel SiSeRO devices.

2020

The beginning of the SKA era : Exciting initial results from the South African SKA pathfinder MeerKAT

Date: 2020-12-24
Speaker: Dr. Viral Parekh (Rhodes University South Africa)
Venue: https://meet.google.com/aev-fbgt-xdk

Abstract

MeerKAT is the South African radio astronomy project developing in the direction of building the ambitious Square Kilometer Array (SKA) radio interferometer. MeerKAT array consists of 64 dishes is located in the Karoo desert, north of the South Africa. MeerKAT is now fully operational and already started to deliver remarkable results both in continuum and spectral line observations at GHz frequencies. In this talk, I will show the initial continuum science results from the MeerKAT high sensitive radio observations. I will also talk about our ongoing galaxy cluster science projects with MeerKAT.

HI absorption associated with a quasar at z~3.5

Date: 2020-11-05
Speaker: Dr. J.N.H.S. Aditya (PRL Ahmedabad)
Venue: https://meet.google.com/aod-mbxm-vmz

Abstract

Studies of HI 21-cm absorption associated with AGNs have been finding a dearth of absorbers in high redshift (z > 1) samples, while there are over 150 such detections in systems at low redshifts. A low HI 21-cm absorption strength is reported in a large and uniformly selected sample of high redshift flat-spectrum sources, which has been attributed to either a redshift evolution in gas properties, or to a high AGN UV and/or radio luminosities. A cut-off UV luminosity (10^23 W/Hz) has been proposed in the literature, above which all the neutral hydrogen in a typical Milky-Way like host galaxy is expected to be completely ionised. In our recent work, we report HI 21-cm absorption, and Lyman-alpha absorption, associated with the high redshift (z~3.5) flat-spectrum radio source 8C 0604+728. The UV luminosity of the source exceeds the above-mentioned cut-off. The detection of neutral gas associated with the AGN provides evidence contrary to the literature hypothesis. In this talk, we discuss the implications of these results.

Intra-night optical variability (INOV) in radio-loud narrow line Seyfert 1 (RL-NLS1) galaxies

Date: 2020-10-22
Speaker: Dr. Parveen Kumar (PRL Ahmedabad)
Venue: https://meet.google.com/iap-bdrh-oug

Abstract

Narrow-line Seyfert 1 galaxies (NLS1s) are active galactic nuclei (AGN) possessing systematically lower black hole masses. Several radio-loud NLS1s (RL-NLS1s) have been detected in gamma-ray energy bands suggesting the presence of blazar-like jets in RL-NLS1s. Relativistic-jets in RL-NLS1s are further confirmed from the high-resolution very large baseline array (VLBA) observations. Therefore, RL-NLS1s present a challenge for the relativistic jet paradigm, which states that powerful radio jets are associated exclusively with very high SMBHs. Given the limitations of radio and γ-ray observations, optical monitoring can be used as a complementary tool to infer the presence of relativistic jets in RL-NLS1s. AGN with relativistic jets aligned close to the line-of-sight are expected to exhibit INOV due to the beaming effect. To probe INOV in RL-NLS1s, we have carried out optical monitoring of a sample of RL-NLS1s. In this talk I shall present statistical results of INOV in NLS1s and their plausible interpretation.

Exploring NGC 6300: A Changing-look AGN

Date: 2020-10-08
Speaker: Dr. Arghajit Jana (PRL Ahmedabad)
Venue: https://meet.google.com/iyq-wgwx-ygf

Abstract

Changing-look (CL) AGNs are known to switch between type-1 and type-2 galaxies. In X-rays, the changing-look events are associated with the changes of flux, spectral shape, and line-of-sight hydrogen column density. Change of accretion states, migrating clouds in the line-of-signt, tidal disruption events, supernovae are believed to be the reasons for the CL events. NGC 6300 is a nearby CL AGN. We study the spectral and timing properties of NGC 6300 using Suzaku, Chandra and NuSTAR data, obtained between 2007 and 2016. From our analysis, we find that the circumnuclear tours is clumpy, and migrating clouds are responsible for change in line-of-sight hydrogen column density. Accretion rate is responsible for the spectral variability. We also find that the torus and AGN both evolve simultaneously between 2007 & 2016.

Particle acceleration in CME shocks

Date: 2020-08-25
Speaker: Ms. Shanwlee Sow Mondal (PRL Ahmedabad)
Venue: https://meet.google.com/vrx-tyqc-ftp

Abstract

Astrophysical shocks are efficient particle accelerators giving rise to power-law energy spectra and magnetic field amplification of the ambient medium. Shocks around supernova remnants are considered to be the sources of galactic cosmic rays. On the other hand, shocks driven by the Coronal Mass Ejections are prominent sources for Solar Energetic Particle events. In situ measurements show rise in particle flux at energies well above the ambient plasma's thermal energy. The fundamentals of the particle acceleration processes active in such astrophysical shock waves are essentially based on the diffusive shock acceleration (DSA) mechanism. Using a sophisticated Magnetohydrodynamics-Particle in Cell method, we have simulated a scenario where the propagation of a CME leads to the formation of a strong shock (of Alfvénic Mach, MA ~ 19). We find that such a strong shock is capable enough to accelerate the SW particles to energies ~ 10 MeV. The dominant mechanism for such acceleration comes out to be the DSA as the particle energy spectra derived from our simulation maintains a slope of ~ -3/2. We also have studied the effect of other weak shocks whose results will also be discussed.

Polarisation measurements of Cygnus X-1 with CZTI on-board AstroSat satellite

Date: 2020-08-24
Speaker: Mr. Abhay Kumar (PRL Ahmedabad)
Venue: https://meet.google.com/rfw-yppa-mdp

Abstract

Cygnus X-1 is a galactic high mass X-ray binary(HMXB). It is a first widely accepted black hole, with a mass of ~15 solar mass and a companion O type star of 27 solar mass. It accretes matter from the stellar wind coming from the companion. Its persistent brightness and proximity to earth(~1.85 kpc) have made it an obvious choice of study by different instruments and in different wavelength regimes. Radiation coming from the astrophysical sources can be characterized in terms of spatial image, energy spectrum, temporal evolution, and polarisation. The first three variables are well studied, but polarisation measurements are done only for few sources due to its photon hungry nature and instrumental limitations. There are different models to explain the source's geometry and emission mechanisms based on the first three variables. Polarisation measurements can break the degeneracy and give a more constraint model to explain the source. It is mainly caused by the scattering and synchrotron emission in astrophysical sources, which depend upon the geometry, magnetic field strength, and structure of the source. It is well established that CZTI, which is meant for spectroscopy and imaging purpose, can be used as Compton polarimeter above 100 kev. By fitting the histogram of azimuthal distribution of the Compton events in the detector by a cos square function gives the orientation of the electric field (Polarisation angle) and polarisation fraction(PF) of the radiation. The analysis procedure is validated by measuring the polarisation of Crab nebula, which is done earlier by Vadawale et.al. Earlier, Bragg polarimeter on-board OSO-8, INTEGRAL/SPI in 250-400kev and >400 kev, and POGO+ a balloon-borne mission in 20-180 kev gave some upper limit of polarisation. We have found the PA equal to 220 degrees in 100-500 kev, which is the same as INTEGRAL measurements. In this talk, polarisation measurements of Cygnus X-1 done using these instruments and what we have measured using the Astrosat-CZTI will be discussed.

Soft X-ray spectroscopy of the small Solar flares by XSM on-board Chandrayaan-2 orbiter

Date: 2020-08-21
Speaker: Mr. Biswajit Mondal (PRL Ahmedabad)
Venue: https://meet.google.com/gpi-kuzq-ojz

Abstract

Solar corona is very hot and hence emit high energy radiation up to a few keV. However, during the massive solar flare, the emitted radiation is extended to a few 10s of keV. Hence the study of soft X-ray spectrum of Solar corona along with the flares could be a diagnostics for the plasma properties. One of the important factors for the coronal plasma is to understand the elemental composition in it. The solar chemical composition is an important ingredient in our understanding of the formation, structure, and evolution of both the Sun and our Solar System. Also, the knowledge of the elemental compositions at different layers of the Sun helps us to improve our understanding of the energy transport from the inner to outer atmosphere and heliosphere of the Sun. Chandarayaan-2 Solar X-ray Monitor (XSM) observes the Sun as a star in the soft X-ray band of 1 – 15 keV to provide spectral measurements at a cadence of one second. In this talk, I will discuss the soft X-ray spectral modeling and the measurement of elemental abundance during a Solar flare.

Broad-band X-ray spectral characteristics of Compton-thick AGN

Date: 2020-08-20
Speaker: Mr. Abhijit Kayal (PRL Ahmedabad)
Venue: https://meet.google.com/afk-pego-zcb

Abstract

The circumnuclear matter surrounding active galactic nuclei (AGN) is thought to be responsible for obscuration and scattering of the primary continuum emission in type-2 AGN, and plays a critical role in accounting for the characteristics of the X-ray spectrum. In recent years, much emphasis has been placed upon Compton-thick AGN (N_H > 1.2 x 10^24 cm^-2) owing to the fact that a high fraction of obscured population of AGN is required to explain the cosmic X-ray background spectrum. We attempt to understand the nature of reprocessing material by modelling the broad-band X-ray spectrum of a nearby Compton-thick AGN namely Circinus galaxy. We find that the broad-band X-ray spectra over 3 – 79 KeV obtained with NuSTAR and AstroSat-LAXPC can be fitted with a model considering reprocessing of X-ray emission from putative torus viewed edge-on with column density much higher than the Compton-thick limit.

Variability of X-ray/UV/Optical emission in Seyfert 1 galaxy Mrk 509

Date: 2020-08-18
Speaker: Ms. Neeraj Kumari (PRL Ahmedabad)
Venue: https://meet.google.com/hxh-qjas-wez

Abstract

Large flux variations in X-rays and UV/optical range, on the timescale of hours to years, are very common in active galactic nuclei (AGNs). There are different proposed physical mechanisms for these variabilities depending on their timescales. To investigate the cause of variation in emitted radiation from different parts of the accretion disc, we have used multi-wavelength observations of Seyfert 1 galaxy Mrk 509 from SWIFT observatory. In this talk, I will be discussing different physical mechanisms such fluctuations in the accretion disk and reprocessing scenario, describing variability. The different techniques have been used for calculating correlation and time-lags between different energy bands. We test the existing assumptions in the standard Shakura-Sunyaev disk which is described by the power-law relating time-lag and the wavelength.

Confirming the suspected symbiotic nature of SU Lyn : A new kind of symbiotic system

Date: 2020-08-13
Speaker: Mr. Vipin Kumar (PRL Ahmedabad)
Venue: https://meet.google.com/koz-hktt-bzt

Abstract

Symbiotic stars (Syst) are interacting binary systems typically consist of a white dwarf(WD) and a cool giant companion. Syst can be revealed through their characteristic optical spectra showing several high excitation emission lines along with Balmer series of hydrogen lines, superimposed on the continuum of a red giant. Though symbiotics have been studied since decades, a recent discovery of SU Lyn - an ordinary red giant from ground based optical observations - in X-rays gave rise to suspicion that it may harbours a hidden WD, thus making it a promising candidate of a new class of symbiotic systems. Using UV spectroscopy from UVIT instrument on ASTROSAT satellite and ground based optical-NIR spectroscopy, We have successfully confirmed the symbiotic nature of SU Lyn. This is, to the best of our knowledge, the first attempt to utilise spectroscopy capability of UVIT. The confirmation of SU Lyn as a new class of symbiotics has opened up an unknown window to discover more objects of this kind, and carries a great significance given that the currently known population of symbiotics are nearly a factor of 1000 less than the predicted one.

Chandrayaan-2 Solar X-ray Monitor: Calibration, Data Analysis, in-flight Performance, and Science Prospects

Date: 2020-08-12
Speaker: Mr. Mithun N. P. S. (PRL Ahmedabad)
Venue: https://meet.google.com/wic-ccnp-qzd

Abstract

Chandarayaan-2 Solar X-ray Monitor (XSM), designed and developed by PRL, observes the Sun as a star in the soft X-ray band of 1 – 15 keV to provide spectral measurements at a cadence of one second. The broadband X-ray spectra with XSM are used to aid estimation of global elemental abundances on the Moon as well as for independent investigations of the Sun. The design features of the XSM such as the use of Silicon Drift Detector, automated filter wheel mechanism, and onboard data processing and intelligence enable XSM to have two unique characteristics: (i) obtain spectra with an energy resolution of better than 180 eV at 5.9 keV; and (ii) maintain this spectral performance over the wide range of incident fluxes expected during different classes of solar flares ranging from sub-A class to X5 flares. XSM has been operational in lunar orbit from September 2019, and the in-flight performance of the instrument so far has been excellent and identical to that on the ground. Particularly, the observations show that the XSM has a very low background as expected and can measure the solar spectrum even during periods of extremely low activity with the flux levels an order of magnitude below the GOES A1 class. In this talk, I will discuss the aspects of ground and in-flight calibration of XSM, data analysis procedure and payload operations center activities, onboard performance, and some specific science cases that can be addressed using XSM observations along with some initial results.

Search for remnant radio galaxies in deep field surveys

Date: 2020-08-11
Speaker: Mr. Sushant Dutta (PRL Ahmedabad)
Venue: https://meet.google.com/zdv-xmaa-dbc

Abstract

The final phase of radio galaxy life-cycle, referred as remnant phase, is being characterized with the reduced AGN activity and stoppage of jets supplying energetic plasma to the lobes. Remnant radio galaxies (RRGs) are supposedly rare, and therefore, statistical study of RRGs is vital to understand jet duty-cycle, and radiative cooling mechanisms in lobes. We have searched RRGs in the XMM-LSS extragalactic field using deep 325 MHz GMRT survey and existing multi-frequency radio surveys. To achieve high completeness we use both morphological criteria as well as spectral criteria, and find an upper limit on the fraction of RRGs to be nearly 7 percent. The low-fraction of RRGs suggests that extended radio emission in lobes tends to fade quickly once AGN-jet activity switches off. In this talk, I shall explain the selection criteria, related biases and characterization of RRGs.

Search for exoplanets: Study of line bisectors from stellar spectra and its relation with PRV measurements

Date: 2020-08-07
Speaker: Ms. Akanksha Khandelwal (PRL Ahmedabad)
Venue: Google Meet https://meet.google.com/gsb-xsyn-rcy

Abstract

Exoplanets are the planets that orbit around stars other than the sun. Since the discovery of the first exoplanet, the field has evolved, significantly and a tremendous amount of research has been going on in the field. Still, the detection or characterization of exoplanets remains important to study them in detail. The most reliable exoplanet detecting method is the radial velocity (RV) method. With the purpose of contributing to this field, a long term program has begun at PRL using the PARAS spectrograph, which is attached to our 1.2 m telescope at Mt. Abu. This program focuses on the search of sub-Neptunes to Jupiter mass planets around late F, G, and K type stars using the highly precise Doppler spectroscopy. We search for the periodic variations in RV data. Sometimes, these periodic variations can be mimicked by magnetic activity induced on the stellar atmosphere or contamination of nearby sources. In order to determine the origin of these RV variations, a technique called line bisector analysis is used. I have developed a line bisector tool for PARAS data. In this talk, I will briefly discuss exoplanets and its detecting methods as well as PARAS and its data reduction & analysis procedure, and the bisector analysis will be discussed in detail.

Observational signature of End-Dominated Collapse in Monoceros R1 Filament

Date: 2020-08-06
Speaker: Mr. Naval Kishor Bhadari (PRL Ahmedabad)
Venue: Google meet https://meet.google.com/ete-cpxi-chj

Abstract

Filamentary features are commonly observed in star-forming regions. They are elongated structures of molecular clouds and often found to nurture the star formation activities along their major axis. The presence of HII regions, embedded clumps, and the clusters of young stellar objects (YSOs) toward the filaments suggests that filaments play a significant role in the star formation processes. To study the role of filaments in star formation, we have carried out an analysis of multi-wavelength data of a star-forming complex Monoceros R1 (distance~760pc). Our observational results reveal the presence of an elongated filament (length ~14 pc). The CO molecular line data confirm the existence of an isolated and single filament, which is traced in a velocity range of [-7.8, 1.3] km/sec. The two previously known star-forming sites IC 446 and IC 447 are found to be located toward the filament's ends. A massive YSO is embedded in IC 446, while IC 447 contains several massive B-type stars. Based on the photometric analysis of point-like sources, clusters of YSOs are traced mainly toward the ends of the filament. Altogether our observational results are consistent with the End Dominated Collapse (EDC) model of star formation in filaments. The observational examples of EDC model are limited in the literature. In this talk, the observational results of our study on Mon R1 region will be presented.

Spectral and timing study of the black-hole binary MAXI J1631-479

Date: 2020-07-30
Speaker: Mr. Sandeep Rout (PRL Ahmedabad)
Venue: Google Meet

Abstract

Low-mass black hole binaries remain in quiescence for most of the time and are only detected when they undergo outburst; thus, becoming bright in the entire electromagnetic spectrum. Most of the knowledge about these systems have come from extensive study of the variation of intensity as a function of energy and time during these sporadic outbursting episodes. In the seminar, after briefly introducing the phenomenology involved, I will present the results of the spectral and timing analysis of a new X-ray binary, MAXI J1631-479, that was discovered in early 2019 and was observed by almost all X-ray telescopes. By tracking the source for the entire duration, we are able to understand the evolution of the source and pin down physical models for timing variability. Using reflection spectroscopy, we could constrain physical parameters like BH spin and inclination of the binary.

The case for Interstellar comets/Asteroids Part II - Detection and Analysis of first IS visitors

Date: 2020-07-28
Speaker: Mr. Aravind K. (PRL Ahmedabad)
Venue: Google meet

Abstract

As described in the previous seminar, Interstellar comets have been a topic of interest from the past years. There have been various theories and speculations regarding the existence of such a class of comets and the probability of their detection in our solar system. It was not until 1993 that a satisfactory result related to the number of comets expected to be detected, in a century, was calculated. Later, in 2017, as a breakthrough finding, came the detection of the first interstellar object 1I/'Oumuamua. Two years later, astronomers were gifted with a direct opportunity to closely study the remnants of the formation of another stellar system by observing the first ever interstellar comet, 2I/Borisov, to be detected passing through the solar system. In this talk, I will discuss in-brief, the properties of both interstellar objects, 1I/'Oumuamua & 2I/Borisov as observed by astronomers across the world and cover in-depth the observation of 2I/Borisov using various techniques by our group in PRL, using MIRO, Abu and HCT, Hanle.

Distance estimation from Gaia parallaxes

Date: 2020-07-23
Speaker: Ms. Namita Uppal (PRL Ahmedabad)
Venue: Google meet

Abstract

Stellar distance is an important parameter in astronomy. Most of the methods used to evaluate distance depend on the stellar type, their luminosity, and age. Parallax is the only method that is applicable to all the stars irrespective of their classification and it depends solely on geometry. The ground-based parallax observations suffer from atmospheric disturbances causing limited precision in the position measurement, which calls for the need to go for space missions. One of the dedicated space based missions - Gaia (successor of Hipparcos) was launched by ESA in December 2013 for precise astrometric measurement. It has measured the parallax of approximately 1.33 billion stars with microarcsec astrometric accuracy. In this talk, I will discuss the problems associated with the usual distance estimation i.e., inverse parallax method and the techniques devised to deal with them in a self-consistent manner using Bayesian inference.

Optical design of the Near IR Imager, Spectrometer & Polarimeter (NISP)

Date: 2020-07-21
Speaker: Ms. Archita Rai
Venue: https://meet.google.com/jqt-pvdj-vqg

Abstract

NISP (Near-Infrared Imager, Spectrometer & Polarimeter) is a multi-faceted instrument for the 2.5 m telescope. The ability to perform imaging, spectroscopy and polarimetry in the wavelength range of 0.8 micron to 2.5 micron in a straight-through optics in a single instrument, makes the optical design interesting. In this talk, I will be discussing about the conceptual background for the optical design of a Near-Infrared instrument and then discuss the optical design of NISP. It encompasses the switching from one mode of operation to another which shall be explained. Apart from the imaging and spectroscopic specifications, the instrument is unique in having a single-shot simultaneous 4 position angle measurement technique for polarisation. The talk will summarize the achieved specifications for the NISP instrument and the efforts incorporated to fix the odd challenges an optical designer has to go through.

The case for Interstellar comets/Asteroids Part I - Speculations and Conclusions

Date: 2020-07-17
Speaker: Mr. Aravind K. (PRL Ahmedabad)
Venue: Google meet

Abstract

After the pioneering work by Oort (1950), it is believed that long period comets (orbital period greater than 200 years) are originated in the Oort Cloud, a hypothetical spherical reservoir surrounding the solar system at about a distance of 1000-200,000 AU. Solar system formation theories suggest that comets were formed in the outer edge of the planetesimal disk, perturbed by the giant planets and thrown into their present location in the Oort cloud. This mechanism was so inefficient that only a fraction of the comets formed were trapped into the Oort Cloud whereas the rest were lost to the interstellar medium. If this physical process of solar system formation is typical, then other stars in the solar neighbourhood are also expected to possess planets and comet like bodies. These systems would also have lost a large fraction of comets into the interstellar medium. Hence there should be a large population of extrasolar comets with composition and density distribution similar to that of the parent star system. For a long time, astronomers have been interested in knowing whether these extrasolar comets can be detected since they are direct opportunities to closely study the remnants of the formation of another stellar system. In this talk, I will discuss in brief, the theories and hypothesis, that have been evolving for a very long time, regarding the existence and possible detection of interstellar comets. In Part 2, I will be discussing in-depth the detection and analysis of 2I/Borisov by astronomers across the world as well as by our group in PRL using MIRO and HCT.

Be/X-ray binary Pulsar 2S 1417-624 : A case study during quiescence

Date: 2020-06-24
Speaker: Ms. Shivangi Gupta (PRL Ahmedabad)
Venue: Google Meet

Abstract

Be/X-ray binary (Be/XRB) pulsars are the major sub-class of High Mass X-ray Binaries (HMXBs). About 67% of the known population of HMXBs belongs to this category. The neutron stars in these systems are powered by the matter accreted from the circumstellar disk of the massive Be companion, at periastron passage. The Be/XRB pulsars show a wide variability in terms of their luminosity. However, the quiescent emission from these pulsars (i.e., emission at extremely low mass accretion rates) is poorly studied as the sources are extremely faint and difficult to observe. Recent observational evidence challenges the present understanding about quiescence emission. In this talk, I will present a case study of Be/XRB pulsar 2S 1417-624 during its quiescence state and discuss the results in light of present theoretical understanding.

Be/X-ray binary Pulsar 2S 1417-624 : A case study during quiescence

Date: 2020-06-24
Speaker: Ms. Shivangi Gupta (PRL Ahmedabad)

Abstract

Be/X-ray binary (Be/XRB) pulsars are the major sub-class of High Mass X-ray Binaries (HMXBs). About 67% of the known population of HMXBs belongs to this category. The neutron stars in these systems are powered by the matter accreted from the circumstellar disk of the massive Be companion, at periastron passage. The Be/XRB pulsars show a wide variability in terms of their luminosity. However, the quiescent emission from these pulsars (i.e., emission at extremely low mass accretion rates) is poorly studied as the sources are extremely faint and difficult to observe. Recent observational evidence challenges the present understanding about quiescence emission. In this talk, I will present a case study of Be/XRB pulsar 2S 1417-624 during its quiescence state and discuss the results in light of present theoretical understanding.

Energy dependent phase resolved Crab polarimetry using AstroSat CZTI

Date: 2020-06-11
Speaker: Ms. Aarthy E. (PRL Ahmedabad)
Venue: Google Meet

Abstract

The Crab pulsar and its nebula is one of the most favorite and well studied Pulsar Wind Nebula (PWN) over all the ranges of the electromagnetic spectrum. Though the field of observational astronomy has evolved tremendously over time with improvements in imaging, timing, spectroscopy, and polarimetry studies, the geometry and the emission mechanism from the Crab pulsar itself are not completely known. Measuring polarization helps in studying the emission process, magnetic field and geometry of source and its surrounding medium. Radio, Optical, and X-ray/Gamma ray polarization of the Crab pulsar and nebula have been reported a number of times. There are various models compatible with observational characteristics, while multiple models explain the features of Crab light curve and spectra. One distinguishing feature of these models is their phase-dependent polarization signature. Phase resolved polarization measurements of the Crab pulsar in hard X-ray regime was reported for the first time by Vadawale et al., 2018 using the CZT Imager on-board AstroSat. A swing in the polarization angle was reported within the pulse period and the off-pulse region was found to have high degree of polarization. Following this, we used ~1800 ks of CZTI Crab data and performed phase resolved polarization over different energy ranges (dynamic and independent from 100 - 380 keV). In this talk I would present the latest results of Crab polarization using AstroSat CZTI.

A heterogeneous aggregate dust grain model. An application to cometary polarization

Date: 2020-05-28
Speaker: Dr. Prithish Halder (PRL Ahmedabad)
Venue: Google Meet

Abstract

In this work we introduce a heterogeneous aggregate dust model in order to replicate the polarization-phase curve observed in comets. Considering the findings of dust morphology from Rosetta/MiDAS, we introduce grain polydispersity of upto ±40nm and fractal dimension less than 2. Also we increase the size of the aggregates by considering hierarchical growth giving rise to hierarchical aggregates. With increasing polydispersity the degree of linear polarization indicates an abrupt reduction in the slope of the polarization-phase curve in the angular range 20 - 90 degrees. On the other hand with increasing size parameter(X), polarization reduces till X=50 and then increases and remains within P_max = 20% - 30% , P_min = 2% and inversion angle around 20 degrees. The polarization-phase curve predicted by the modeled heterogeneous, polydisperse aggregates agrees well with the observations for 1P/Halley and 67P/C-G. Finally the heterogeneous dust aggregate model indicates that grain polydispersity, composition and size of an aggregate shapes the observed polarization-phase curves for various comets.

Triggering of a solar filament eruption and associated flare

Date: 2020-03-12
Speaker: Mr. Suraj Sahu (USO, PRL Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Solar filaments are large magnetic structures confining cool (T ~ 10^4 K) and dense (ne ~ 10^17 m^{-3} ) plasma in the hot solar corona. Typically, the filament plasma is 100 times cooler and denser than its coronal surroundings. Depending upon the type of magnetic environment, in which filaments form, they are classified as: active region filament (ARF), intermediate filament (IF), and quiescent filament (QF). Study of active region filaments is key to understand the evolution of magnetic fields of active regions and its role in powering solar eruptive phenomena, viz., flares and coronal mass ejections (CMEs). With these motivations, we study the eruption of an active region (AR) filament from AR NOAA 12371 on 22 June 2015, which led to a major M6.6 solar flare and a halo fast CME. The study utilizes data from Atmospheric Imaging Assembly (AIA) and Helioseismic Magnetic Imager (HMI) on board Solar Dynamics Observatory (SDO), Big Bear Solar observatory (BBSO), and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The event exhibited an active pre-flare phase during which a hot EUV coronal channel (co-spatial with filament channel) was in build-up stage and displayed hard X-ray emission up to 25 keV. As such, this is the first evidence of HXR coronal channel. The Non-linear-force-free-field (NLFFF) modeling of coronal magnetic field exhibited a magnetic flux rope (MFR) oriented along the polarity inversion line (PIL) and co-spatial with the coronal channel. We observe significant changes in the AR’s photospheric magnetic field at the activity site during an extended period of about 42 hours in the form of rotation of sunspots, moving magnetic features, and flux cancellation along the PIL. Prior to the flare onset, the hot channel underwent a slow rise phase (~14 km/s) for about 13 min which is followed by a fast rise (~ 90 km/s). The slow to fast transition of the hot channel precisely divides the pre-flare and impulsive phase of the flare which points toward the feedback process between the early dynamics of the eruption and the strength of the flare magnetic reconnection.

Source extraction algorithms for deep radio surveys

Date: 2020-03-05
Speaker: Mr. Sushant Dutta (PRL Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The source extraction in deep radio surveys pose challenges as, unlike optical/IR surveys, the PSF and the background RMS vary across the image. Since the source density is large in deep radio surveys, there is always a chance of source confusion, which in turn affects the background RMS estimation. Rudimentary source extraction algorithms available in the literature are inefficient to produce a complete and reliable source catalogue, with accurate astrometry and flux estimation, in case of diffuse and extended radio sources. These factors can be taken care of in Python Blob Detection and Source Finder (PyBDSF) algorithm developed mainly to serve the need of low-frequency radio surveys. I shall discuss the working principle of PyBDSF and the results of source extraction algorithm applied to deep 325 MHz GMRT image.

High Energy Emission from Pulsars with AstroSat-LAXPC

Date: 2020-02-13
Speaker: Dr. Varun, Raman Research Institute, Bangalore

Abstract

Astrosat is India's first dedicated multi-wavelength space observatory. The Large Area X-ray Proportional Counter (LAXPC) is one of the major payloads on AstroSat, which covers a 3-80 keV energy range. The three LAXPC units on AstroSat have a large effective area which gives it very good timing and moderate spectral resolution. Neutron stars have the most powerful magnetic field strength of 1012-1015 G. One interesting manifestation of the strong magnetic field is the cyclotron resonance scattering feature (CRSF) which is seen in the spectrum of accretion powered X-ray pulsars. I present the results of pulse-phase-resolved spectroscopy of the continuum and CRSF parameters of two high mass X-ray binaries (HMXB) 4U 1538-52 and 4U 1907+09 using Astrosat-LAXPC observations. For both the sources, the cyclotron line parameters show a strong pulse phase variation. For 4U 1538-522, we confirm a long term increase in the centroid energy of the CRSF. For 4U 1907+09, intensity resolved spectral analysis of the initial flare in the light curve shows that the CRSF parameters do not change with a change in luminosity by a factor of 2.6. I also present some results on the high energy emission of four isolated neutron stars with AstroSat-LAXPC observations. This study is a demonstration of methodologies to handle these sources and the capability of this instrument for them.

Understanding the origin of the solar wind with Solar Orbiter, DKIST and Aditya-L1

Date: 2020-01-30
Speaker: Dr. Giulio Del Zanna (University of Cambridge, UK)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

One of the main science questions in solar physics is how and where the solar wind is originating and accelerates. Together with Parker's Solar Probe, the main upcoming ESA solar mission, Solar Orbiter, is tasked to address this science question. I will briefly describe the suite of remote-sensing instruments (in particular the EUI imager, the PHI magnetic imager, the SPICE spectrometer and the METIS coronagraph) built to support the in-situ ones, to help locating the source regions of the solar wind. I will then briefly describe the DKIST near-infrared (NIR) coronagraph. DKIST is the first large-scale solar telescope providing unprecedented observations of near-infrared forbidden lines in the outer corona, to measure magnetic fields, line widths, densities etc. Together with the Proba-3 and the VELC coronagraph, on board Aditya-L1, the first major Indian mission to study the Sun, they will provide important information about the outer corona, in synergy with the Solar Orbiter observations. I will describe the complexities and advantages in using the NIR lines to measure densities, chemical abundances and temperatures.

2019

Sparse star clusters and photometric metallicity maps of the Magellanic Clouds

Date: 2019-12-24
Speaker: Dr. Samyaday Choudhury (Macquarie University, Sydney, Australia)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

I will present our studies directed towards two frontiers in the Magellanic Clouds (MCs, the LMC & SMC), a pair of interacting galaxies near the Milky Way (MW) : (1) Sparse star clusters - Our work on sparse star clusters in the LMC is aimed to increase our understanding of such objects using deep Washington photometric data of 45 star clusters. The sizes and masses of these inconspicuous clusters emphasizes that the LMC has a significant population of clusters, which are similar to the open clusters in the Milky Way (MW). Motivated by the above finding, a larger team is looking into the low mass open cluster like systems in the LMC & SMC using existing large area surveys (e.g. OGLE III). (2) Radial metallicity gradient (MG): The spatial distribution of heavy metals within a galaxy is an important indicator of its evolution and interaction history. The MCs are perfect laboratories to investigate radial MGs in a different environment w.r.t the MW (in terms of gravitational potential & metallicity). We have created first of its kind high-spatial resolution metallicity map by combining large-area optical photometric surveys (MCPS and OGLE III) with spectroscopic data of Red-Giant-Branch stars. These maps reveal the metallicity trend across the inner ~ 5 and 2.5 deg. field region of the LMC and SMC respectively. The MG of the LMC is almost constant within the bar region and falls off beyond that, indicating an active bar in the past. Whereas, the presence of any MG in the SMC is still debatable, and requires spatially larger data set to make significant advances. Presently, we are using the VMC-VISTA survey (a near-infrared public survey by the ESO) to unravel the metallicity trend within the SMC, covering 2-3 times larger spatial area (bar and regions of interaction) compared to our previous studies.

Investigating the cosmological principle using large surveys

Date: 2019-12-19
Speaker: Prof. Ashok Singal (PRL Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

According to the cosmological principle (CP), the universe is homogeneous and isotropic, without any preferred directions. The cosmic microwave background radiation (CMBR) shows an isotropic distribution, except for a dipole anisotropy, ascribed to a solar system peculiar velocity. The NRAO VLA Sky Survey (NVSS), comprising 1.8 million radio sources, has shown a dipole asymmetry corresponding to a velocity ~4 times the CMBR value, a result confirmed later by many independent groups. Further, in the TIFR GMRT Sky Survey (TGSS), comprising 0.62 million sources, a very significant (>10 sigma) dipole anisotropy, amounting to a velocity ~10 times the CMBR value, is detected. However, the direction of motion in both cases has turned out to be along the CMBR dipole. More recently, a homogeneously selected DR12Q sample of 103245 distant quasars has shown a redshift excess along the CMBR dipole direction, implying a velocity ~6.5 times in a direction opposite to the CMBR dipole. Since the solar system peculiar velocity should not dependent upon the specific data or technique used to determine it, an obvious inference could be that the discrepancies in dipole anisotropies seen in different surveys indicate the presence of a preferred cosmic direction (axis!), violating the CP, a cornerstone of the modern cosmology.

X-ray Relativistic Reflection & Testing Strong Gravity

Date: 2019-12-12
Speaker: Dr. Kishalay Choudhury (IUCAA Pune)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The first part of the talk presents results on the first, well-structured systematic study on the most up-to-date relativistic X-ray reflection code relxill for the dimensionless spin (a∗) parameter of astrophysical black holes, with spectral analysis of high-) parameter of astrophysical black holes, with spectral analysis of high- quality simulated data, emphasizing on only unabsorbed relativistic smearing and the detector resolution with NuSTAR. Yields even at high signal and low background seem to be subjective to fitting biases and the treatment of data, when checked against a similar work from literature. Nevertheless, a∗) parameter of astrophysical black holes, with spectral analysis of high- > 0.8 seems to be best recovered under the averaged conditions. The second part jumps into utilizing the knowledge gained from the first half, testing Einstein’s strong gravity for the Kerr hypothesis in 4-dimensional spacetime around the supermassive black hole of the narrow-line Seyfert 1 galaxy Mrk 335 using the only available, high-signal Suzaku data for a low-reflection estimate with the initial release of the now-publicly- available non-Kerr extension to relxill (relxillnk), developed at the astrophysics group in Fudan University. Results tend to show weak constraints for a∗) parameter of astrophysical black holes, with spectral analysis of high- against two of the leading-order deformation parameters α13 and α22 of the Johannsen metric, recovering the Kerr metric at 1σ, and carry no weight or physicalvalidity on solutions deviating from the Kerr geometry. This has been confirmed with a later, more sophisticated MCMC error analysis of the data with more up-to-date code and grids to examine the persistence of spurious solutions. A hybrid model, however, has been proposed for the first time for a high-flux state in this AGN, even with fits using the Kerr base model.

Investigating Particle Acceleration in the Wolf-Rayet bubble G2.4+1.4

Date: 2019-12-05
Speaker: Ms. Prachi Prajapati (PRL Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The supersonic winds produced by massive stars carry a large amount of kinetic power. In numerous scenarios such winds have been proven to produce shocks in which relativistic particles are accelerated emitting nonthermal (NT) radiation. This seminar will be focused on the first detection of NT emission from a single stellar bubble, G2.4+1.4, associated with a WO-spectral type Wolf-Rayet star, WR 102. Based on the present literature, massive star WR 102 is the hottest star known today with the surface temperature of 200,000 K. Upgraded Giant Metrewave Radio Telescope (uGMRT) observations were carried out for this investigation. The fraction of the available kinetic wind power of stellar winds that is converted into cosmic-ray acceleration is estimated to be of the order of a few percent. This finding constitutes an observational breakthrough and gives new insight on the NT physical processes taking place in the environments of isolated massive stars. In particular, the results show that non-runaway isolated massive stars are capable of accelerating relativistic particles and are therefore confirmed as sources of Galactic cosmic rays.

Challenges in the calibration of astronomical X-ray instruments

Date: 2019-11-22
Speaker: Dr. Sunil Chandra
Venue: Thaltej Seminar Room (113/114)

Abstract

The calibrations of X-ray instruments dedicated to astronomical studies comprise a number of challenging tasks. The poor knowledge of optics and detector parameters may end in surprisingly inaccurate scientific outcomes. I have been working for the Soft X-ray Telescope (SXT) aboard AstroSat for the last 5 years. Being part of the instrument team even before the launch, I have developed a level of understanding about the basic system parameters such as effective area of the system, charge transfer in-efficiency, detector background etc and changes in these with mission life. During this seminar I would like to share my experiences of various calibrations of SXT and also will update you with the current status of SXT calibration.

Accretion-Ejection Coupling in Black Holes

Date: 2019-10-21
Speaker: Mr. Arghajit Jana (Indian Centre for Space Physics, Kolkata)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The coupling between accretion disk and jet is well known. Observationally, strong signatures of disk-jet connections have been seen for many black hole binaries. The nature and evolution of jets depend on accretion flows around black holes. Correlation between X-ray and radio fluxes are also visible, which indicate the coupling between accretion disk and jet. Here, we discuss about the observational evidences of the disk-jet connections in the two-component advective flow (TCAF) paradigm. With TCAF, we have separated the jet and accretion disk components from the observed X-ray. We have studied the properties of the X-ray jet for a few black holes and the results will be discussed in the seminar.

Cold gas in AGN host galaxies

Date: 2019-10-10
Speaker: Dr. J.N.H.S. Aditya (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Probing the distribution and kinematical properties of cold gas present in the vicinity of active galactic nuclei (AGN) is critical for assessing the nature of interactions between the AGN and its host galaxy, thereby for understanding the AGN-host galaxy co-evolution. Cold atomic gas (HI) present in the surroundings of AGN can be probed via HI 21 cm absorption, that is an efficient technique for observing systems at high redshifts, z > 0.3. Our earlier studies have established that samples of compact radio sources at high redshifts, z > 1.0, have a significantly low HI 21 cm absorption strength compared to samples at lower redshifts. We find that this discrepancy could be caused by multiple factors: (1) a redshift evolution in the AGN host galaxies, (2) a varied gas covering factor, (3) or a systematic bias in the AGN ultraviolet and/or radio luminosities. We use our recent uGMRT observations, and similar searches available the literature, with the aim to identify the dominant cause among the above, if any. In the talk I will discuss about our recent new detection of HI 21 cm absorption at the highest redshift to date, z = 3.53. The source has an ultraviolet luminosity of 1023 W/Hz, a cut-off above which all the neutral hydrogen in the host galaxy is expected to be completely ionized. Our detection of HI 21 cm absorption in this system is thus a contrary to this hypothesis. Finally, I will talk about the importance of near-IR spectroscopic observations to probe the cold molecular gas in the AGN host galaxies.

Spectral study of X-ray binary MAXI J1659-152

Date: 2019-08-16
Speaker: Mr. Sandeep Kumar Rout (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Black holes with a low mass star as a companion (LMXB), spend most of their life in quiescence. They become visible in the X-ray sky only during violent episodes of outbursts triggered by an instability in the accretion disk. Much of the information about black holes has been acquired by studying the emission from the accretion disk and its surrounding during these outbursts. In the seminar, I will present the results of a broad-band spectral analysis of an LMXB MAXI J1659-152 during its September 2010 outburst. One important highlight of this work is the unambiguous detection of a retrograde spin of the black hole. The possible implication of spin on the apparent faintness of some BHBs will also be discussed.

X-ray Polarisation in general, its measurements techniques and hard X-ray Spectro-polarimeter

Date: 2019-08-14
Speaker: Mr. Abhay Kumar (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Polarisation is the fundamental property of light. Polarisation studies can help in verifying different models given for astrophysical systems like in AGN, GRBs, black-hole binaries etc. to explain their geometry, strength of magnetic field etc. It can help in understanding the environment, geometry and magnetic field in the astrophysical systems and will be able to get the more correct model. I will talk about the basics of polarisation, polarisation in X-ray and its different measurement technique. I will talk about what is the importance of hard X-ray polarimeter and my work on the hard X-ray Spectro-polarimeter.

Validation of Hard X-ray Off-axis polarization capability of Cadmium Zinc Telluride Detector

Date: 2019-08-01
Speaker: Ms. Aarthy Essakiappan (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Ever since the launch of Uhuru in 1970, the field of X-ray astronomy has grown tremendously over the past five decades. Though X-ray imaging, spectroscopy and timing studies have vastly advanced both over soft and hard X-ray regimes, the field of X-ray polarimetry is still under explored due to the difficulties in measuring X-ray polarization. The prime reason is, measuring polarization requires a large number of photons from the source. In case of hard X-rays, there has been no focal plane polarimeters so far, and only open detectors have been used to detect polarization. Cadmium Zinc Telluride Imager (CZTI) is one of the five payloads on-board AstroSat which is capable of doing imaging and spectroscopy over 20 - 200 keV and is proven to measure on-axis polarization over 100 - 400 keV. Besides on-axis sources like Crab and Cygnus X-1, CZTI have been detecting Gamma Ray Bursts (GRBs), which by its isotropic nature occur from any random direction in the sky. This creates a great opportunity to measure Hard X-ray GRB polarization (for which very less reliable results exist so far) using CZTI. Hence it is important to validate that CZT detectors could be used as hard X-ray polarimeter for GRBs. In my talk I would present the work done using the QM of AstroSat CZTI at TIFR, complimented by Geant4 simulations to validate the off-axis polarization capability of CZT detectors

Development of FOSC Instrumentation for PRL Telescopes : Commissioning of MFOSC-P and Optical Design of MFOSC-EP

Date: 2019-07-31
Speaker: Mr. Vipin Kumar (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

FOSC (Faint Object Spectrograph and Camera) series of instruments are one of the most general purpose instruments on optical telescopes around the world. Given their ability to do imaging, spectroscopy and polarimetry within a single optical chain, they are highly desired for any optical observatory. Mt. Abu Faint Object Spectrograph and Camera-Pathfinder (MFOSC-P) is one such instrument which has been recently developed and commissioned on PRL 1.2m telescope; while MFOSC-Echelle Polarimeter (MFOSC-EP) is the next instrument, currently under design, for upcoming PRL 2.5m telescope. In this talk, we shall discuss the development of MFOSC-P including its commissioning observations and some of the on-sky characterization results, showcasing the science capabilities of the instrument. In the latter part of the talk, we shall discuss the proposed specifications and optical design of next instrument-MFOSC-EP. While retaining the FOSC functionalities, MFOSC-EP also consists of an intermediate resolution spectro-polarimetry mode which makes it a highly versatile instrument for upcoming 2.5m telescope. We shall summarize our efforts to overcome some of the challenging design problems with its optical design and expected performance.

2018 giant outburst study of Be/X-ray binary pulsar 2S 1417-624

Date: 2019-07-29
Speaker: Ms. Shivangi Gupta (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The accretion powered X-ray pulsars are rotating neutron stars that emit X-rays as a result of mass accretion from their optical companion. They are considered to be an ideal astrophysical laboratory to test the fundamental properties of matter under extreme conditions. During the process of accretion, the interaction between pulsar's magnetic field and accreted gas results in the formation of a column like structure onto the poles of neutron star that acts as the source of X-ray emissions. This accretion column has a complicated geometry, hosting numerous complex processes that shapes the broad-band continuum of these objects. In this talk, I will discuss about these mechanisms, emission beam patterns, and the properties of pulse profiles. A case study of the pulsar 2S 1417-624 during its recent outburst in 2018 and the implications of our study would also be presented in detail.

Interstellar Polarisation: Grain alignment theories and polarisation study towards high opacity LDN clouds

Date: 2019-07-25
Speaker: Ms. Archita Rai (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Interstellar polarisation at optical and near- infrared wavelength bands is thought to arise when light from the distant stars passes through the interstellar medium consisting of asymmetric dust grains aligned with the magnetic field. Different theories have been developed to explain the process of grain alignment. They depend upon the sizes of the grain particles, the refractive index of the grains and the wavelength of observations. A study of the wavelength dependence of polarisation gives us insight into these features for the interstellar medium. The talk will mainly focus on the theories dealing with interstellar polarization by light passing through aligned grains. Some results from an ongoing work on Lynds Dark Nebulae (LDN) would also be presented.

Search and characterization of relic radio galaxies in deep fields

Date: 2019-07-18
Speaker: Mr. Sushant Dutta (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The typical morphology of a radio galaxy consists of a core, pair of jets, lobes and hot-spots. The phase of radio galaxy evolution after the jets have switched off, is referred as remnant phase. The remnant phase is not well understood due to fact that it is short-lived and only few relic radio galaxies are known. We, therefore, have tried to improve the statistics on radio galaxies that have ceased to be active, or are intermittently active. I shall talk about the search for relic radio galaxies in the VLA-VIMOS VLT Deep Survey (VLA-VVDS) field using deep multi-frequency radio observations.

Modelling the solar and stellar magnetic cycles.

Date: 2019-07-17
Speaker: Dr. Bidya Binay Karak (IIT, BHU)
Venue: Thaltej Seminar Room (113/114)

Abstract

The sun and many other low-main sequence stars have active magnetic fields. The solar magnetic field reverses its polarity in about 11 years, and sunspots are the regions of intense magnetic field seen on the solar surface. The number of sunspots varies cyclically with a period of about 11 years which is popularly known as the solar cycle. However, this cycle is not regular; it has variation within a cycle as well as cycle-to-cycle. In this presentation, I will highlight my contribution in explaining these irregular features of the magnetic cycles of the Sun and other stars. Magnetohydrodynamics (MHD) dynamo process is believed to be responsible for such variation. The fluctuations in the so-called Babcock-Leighton process in generating a poloidal field in the solar dynamo, primarily through the scatter in the sunspot tilt, produce variation in the poloidal field and eventually cause irregularities in the solar cycle. I will further highlight our effort in explaining the features in magnetic cycles in other slowly rotating stars.

X-ray studies of GHz-peaked-spectrum (GPS) radio galaxies

Date: 2019-07-11
Speaker: Mr. Abhijit Kayal
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The GHz-peaked-spectrum (GPS) radio sources, a subclass of radio galaxies, are characterized by a convex radio spectrum peaked around 1 GHz. GPS radio galaxies exhibit compact radio size (~ 10 - 1000 pc) and are believed to represent the early stage of radio galaxy evolution. X-ray studies of GPS sources can help us in understanding the nature of environment around AGN and conditions during early phase of AGN activity. We have carried out X-ray spectral modeling of a sample of GPS sources and found that their X-ray spectra are characterized either by absorbed power law with high column density, or relatively flatter power law with Fe K_alpha emission line of high equivalent width. Hence, we conclude that GPS sources reside in dense circumnuclear environment. In this talk I shall also emphasize the need of hard X-ray spectral studies of GPS sources.

Understanding spectral observations of the Sun using numerical simulations

Date: 2019-07-04
Speaker: Shanwlee Sow Mondal
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Spicules are the jets of cold plasma that are chromospheric in origin. Multiple high-resolution imaging and spectroscopic observations have revealed the ubiquitous presence of spicules, especially the type-II spicules, in the solar corona. Such observations have claimed that the spicules play an important role in providing mass and energy to the solar corona and also help in maintaining its million-degree Kelvin temperature. We numerically simulate the propagation of such cold jet in a gravitationally stratified coronal magnetic structure. The simulation shows the heating of the tip of the spicule to million degree Kelvin due to the shock formation. The analysis of the synthetic spectral lines, constructed from the simulation data, lead us to conclude that probably the spicules provide very less plasma to the solar corona.

Comet Wirtanen: Initial target of the Rosetta Mission

Date: 2019-06-24
Speaker: Dr. Kumar Venkatramani (PRL Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Comets carry a significant amount of pristine material from the early solar system and become relatively bright when they approach the Sun. They appear brighter if they pass close to the Earth. Such an occasion gives us an opportunity to follow and probe these objects in detail. One such close approach of a comet happened in December 2018. Comet 46P/Wirtanen, the initial target of the Rosetta mission, had its closest approach to Earth (considering all its previous and future apparitions) on 15th of December 2018, three days after its perihelion. This comet was spectroscopically monitored using the low resolution spectrograph LISA on the 1.2m telescope at Mount Abu. High resolution spectra of this comet were obtained at various epochs using the the Hanle-Echelle Spectrograph mounted on the 2m Himalayan Chandra Telescope in Hanle. The comet exhibited various molecular emission lines like C2, C3, CN, NH2 as seen in the low resolution spectra. Many of the ro-vibrational lines of these molecular species have been identified in the corresponding high-resolution spectra. Many of the lines remain unidentified, due to the lack of an exhaustive line lists of various emission lines in a comet spectrum. Although each comet's composition might be different, there is a general pattern in which these emissions arise in various comets originating from different cometary reservoirs. In order to study the behavior of a comet and the evolution of its molecular emissions, we need to monitor the comet, as it travels past its perihelion. Some of the results from the observations of comet 46P will be discussed in this talk.

Evolution of the Solar System and the importance of studies of the minor bodies

Date: 2019-06-20
Speaker: Mr. Aravind K. (PRL Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Our Solar System evolved from various processes like, formation of Sun, accretion of matter in a protoplanetary disk to form solid planets, gas giant planets, ice giant planets, asteroids, comets, Trans-Neptunian objects etc. There are Solar System formation models which say that the giant planets were at much more closer distance to the Sun, during formation, than at present time. This shift in semi major axis of the orbits happened due to various chaotic processes like resonance capture and exchange of angular momentum with planetesimals. These processes created drastic changes resulting in the current orbits of the planets and confinement of the asteroid belt or various cometary reservoirs. The objects in the asteroid belt between Mars and Jupiter are rocky materials since they could not preserve ice at such close distance from Sun. Whereas, objects in the Kuiper belt or the Oort cloud were formed at larger distances due to which the molecules have been preserved in its pristine form. My talk will hence try to uncover some of the mysteries and theories behind the evolution of our Solar System and therein emphasize on the importance of studies of the Minor bodies.

Hard X-ray Optics for Focusing X-ray Telescope

Date: 2019-06-19
Speaker: Mr. Biswajit Mandal (PRL Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Focusing optics in hard X-rays (10-100 KeV) require multi-layer coatings. In this framework we are developing the hard X-ray mirrors based on RF Magnetron Sputtering technique. Hard X-ray mirrors consists of multilayer coatings with alternate layers of high Z and low Z materials with typical thickness of 10-100 Angstrom. Coatings with equal spacing are effective for discrete energies but the thickness of the layers can be varied over different layers to obtain broader energy range. Design of such dept requires optimization of coating materials, number of layers, their thicknesses, roughness etc. IMD is a widely used software for such optimization, however, being an IDL based GUI software, it has some limitations in terms of iterative scripting capabilities or complex fitting. In this context we have developed alternate implementation of the Fresnel formalism to calculate reflectivity of multi-layer coatings which can be used with the standard spectral fitting software such as ISIS or XSPEC. In this talk, I will give a broad overview of hard X-ray optics for focusing telescope and the technique to form the hard X-ray mirrors and theoretical modeling of the mirrors.

X-ray Studies of Type 1 Active Galactic Nuclei

Date: 2019-05-31
Speaker: Ms. Neeraj Kumari (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

An active galactic nucleus (AGN), as its name suggests, is a compact region at the center of a galaxy which outshines the rest of the galaxy. It is strongly believed that the central super massive black hole (mass range between 10^6-10^10 solar mass) accretes matter from its surrounding medium and converts the gravitational potential energy into radiation. These are the most luminous persistent sources found in the Universe which emit in the entire range of electromagnetic spectrum. AGNs are classified broadly into radio-quiet and radio-loud AGNs depending on their emission in radio. In this talk, I will give a broad overview of AGNs, specifically my area of interest, in this vast field. I will discuss about X-ray properties of Seyfert galaxies which generally fall under the category of radio-quiet AGNs. The UV/X-ray spectrum of a Seyfert galaxy exhibit the following features apart from power law continuum: A Compton hump above 10 keV, a broad/narrow Fe k-alpha line at 6.4 keV, a soft X-ray excess below ~1 keV and a big blue bump (BBB) in UV range. Although the origin of the soft excess is still unclear, there are different physical models which successfully explain soft excess in one or the other Seyfert galaxies and quasars. The X-ray spectrum of Seyfert galaxies also shows signature of warm absorbers. In this talk, I will discuss the current understanding on the origin of various spectral components.

Investigation of a long duration X-ray flare on an active RS Cvn-type eclipsing binary SZ Psc

Date: 2019-05-16
Speaker: Dr. Subhjeet Karmakar (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

In this talk, I will discuss about a long duration X-ray flare observed on an partial eclipsing RS CVn-type binary SZ Psc. The flare was detected by the Neil Gehrels Swift Observatory on 2015 January 15 simultaneously using BAT, XRT, and UVOT. The SZ Psc system consists of a spotted chromospherically active ptimary K1 IV subgiant and a less luminous inactive F8 V secondary companion. The primary subgiant is filling 80-90% of its Roche lobe. With a duration of >1.3 days, this flare is one of the longest duration X-ray flares ever observed. The exponential rise and decay time of the flares were derived to be 3.5 and 5.5 hr, respectively. The peak X-ray luminosity in 0.3-10 keV energy band reached to a value of 4.8 x 10^{33} erg s^{-1}, which is 89 times more than that of the observed minimum value. Spectral analysis indicates a presence of two-temperature corona (11 and 49 MK) near the secondary eclipse whereas at the flare peak the temperature is 258 MK. The peak stellar abundances were derived to be 0.7 times more than solar abundances, which is also 10 times more than that of the minimum abundance observed on SZ Psc. The flare energy estimated to be ~1.98 x 10^{36} erg, and it can also be categorized as a Superflare. The total magnetic field estimated to produce the flare is 1.5 kG. Such large magnetic field at the coronal height is probably due to the presence of extended convection zone of the sub-giant and the high rotational velocity of this tidally locked system.

Spectroscopic and photometric study of cool and evolved stars

Date: 2019-05-09
Speaker: Supriyo Ghosh
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The cool and evolved stars with low-to intermediate-mass (0.8–8 Msun) lie in the Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) in post-main-sequence evolutionary phase. Precise estimation of fundamental parameters of these stars is very important to characterize the stellar populations in the galactic and extra-galactic environments. The atmospheres of AGB stars are strongly affected by the radial pulsation, and therefore, the time-dependent spectroscopy provides the understanding of ongoing physical processes in its atmosphere. I shall talk about the precise estimation of fundamental parameters from dominant spectral features of K–M giants derived by constructing a new near-infrared (1.50–2.45 μm) spectral library of 72 K–M giants using TIRSPEC instrument on 2m HCT. We also derive new empirical correlations between fundamental parameters and 12CO bands strength and study their relative effectiveness for the estimation of parameters. The presentation focuses on the characterization of a red optical transient, which was found as an O-rich (C/O<1) Mira variable, and a poorly known OH/IR star, using long-term optical/near-infrared photometric and spectroscopic observations.

Spectral and timing evolution of long spin-period X-ray pulsars : Case study of 4U 2206+54

Date: 2019-05-07
Speaker: Dr. Prahlad Epili
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

In the frame of traditional scenario of spin period evolution in accretion powered X-ray pulsars, the spin down of neutron star (NS) pulsations beyond ~500s is very rare. However there are a few slow X-ray pulsators residing in binary stellar systems that show pulsations of the order of ~1000’s of seconds. To begin with the evolution of such slow pulsations require a magnetar field strength (~10^14 Gauss) of the NS. In order to understand the such peculiar slow pulsations in X-ray pulsars, we have studied the the X-ray timing and spectral properties of one such long-period (i.e ~5550s pulsation) X-ray pulsar in 4U 2206+54 using our recently proposed Astrosat observations. The timing studies indicate that the pulsar is showing a recent spin-up trend after its prolonged spin-down episodes. Alongwith the associated spectral evaluation the discussion would include the spin-evolution of very slow pulsation accreting neutron stars in binary systems reminiscing as accreting magnetar candidates.

A comprehensive study of comet dust at different heliocentric distances for varying radiation pressure.

Date: 2019-04-18
Speaker: Dr. Prithish Halder
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The study of light scattering properties of comet dust aggregates (0.7μm ≤ RC ≤ 2.0μm) with a wide range of porosity (P = 0.59 to 0.98) has been carried out using the High-Performance Computation (HPC). The results indicate that, when the porosity of the aggregates decreases, keeping characteristic radius RC same for all structures, there is an enhancement in the negative polarization branch which is accompanied by a substantial increase in the anisotropies present in the material. At the exact back-scattering region, the anisotropies are found to be linearly correlated with the porosity of the aggregated structure. Dust in the interstellar medium manifests in the form of extinction and polarization of starlight. To study this, I had carried out polarization observations of the dark cloud CB26 using 1.04-meter telescope at ARIES, Nainital. It has been found that the local magnetic field of the cloud is almost aligned with the Galactic magnetic field. Additionally, the extinction map reveals the presence of an active central core and a second low dense core. The visual extinction of field stars background to CB26 is estimated from 2MASS and Gaia archives using the Near-Infrared Technique. Details of the above studies will be discussed in this seminar.

Where the energy during solar flare is released?

Date: 2019-03-07
Speaker: Dr. Susanta Kumar Bisoi
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

An outstanding question in solar physics is how and where the large amount of energy (up to 10^33 erg) is released during solar flare? The energy released during flare accelerates non-thermal electrons in solar corona which propagate along coronal magnetic field lines as electron beams and produce short-lived radio emission that appear as rapidly drifting structures in a radio dynamic spectra. They are known as Type III radio bursts. Such bursts are commonly associated with electron acceleration and flare energy release, particularly those Type III bursts identified in the decimetric frequency range (400-2000 MHz). Coronal X-ray and extreme Ultra-Violet observations have shown evidences of such electron acceleration. However, the observational evidence of the location of electron acceleration and so the flare energy release is still remained elusive. The ultra-high cadence radio dynamic imaging spectroscopy of Miangtu Spectra Radio Heliograph (MUSER), a radio interferometer located in China that operates in the decimetric frequency range of 400-2000 MHz, can be effectively used to solve this mystery. I will present in my talk MUSER imaging spectroscopy observations of such Type III decimetric bursts observed during flares with high spectral (25 MHz), spatial (1.3 to 50 arcsec) and ultra-high temporal (25 ms) resolution, and discuss how we can use the high resolution and high dynamic range radio imaging of Type III bursts to find the location of electron acceleration, and in turn, the location and nature of flare energy release.

Understanding supernovae diversity and exploring the rare and unusuals

Date: 2019-02-21
Speaker: Dr. Subhash Bose
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Supernovae (SNe) are among the most energetic and luminous events in the cosmos, and thus they are the important probe at extragalactic scales. SNe are produced from a wide range of progenitor and host environments, and so their observed properties are also diverse. Despite several decades of studies of these objects, various aspects are still poorly understood and highly debated. With the emergence of several all-sky and untargeted surveys, we are now able to find several extreme and unusual SNe which were previously unknown and challenges our present understanding of these objects. It is important to examine the rare and peculiar behaviors of these SNe to find clues to the missing links in our present understanding. With a growing sample of large number of observed SNe, It is also important to re-visit existing taxonomy schemes. In this context, I will talk about ASAS-SN survey and some of the rare and very unusual SNe discovered.

History of Astronomy in TIFR and making of AstroSat mission

Date: 2019-01-10
Speaker: Prof. R.K. Manchanda
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

N/A

2018

Density turbulence in the solar wind using low frequency angular broadening observations

Date: 2018-11-15
Speaker: Dr. K. Sasikumar Raja
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Various remote sensing observations have been used so far to probe the weakly compressible density turbulence in the solar wind. Using the angular broadening observations of radio celestial point like sources, we have studied the various turbulent parameters in the solar wind: anisotropic broadening, amplitude of density turbulence, density fluctuations, proton heating rate and the dissipation scales. For this study, we used the observations of Gauribidanur radioheliograph, Very Large Array and other historical observations carried out during 1952-2017. In this talk, I will discuss, how these parameters vary with heliocentric distance range ~2-40 Rsun and the solar cycle. The newly launched Parker Solar Probe may provide valuable insights in understanding these long standing issues of the solar wind.

Gamma-ray emitting narrow-line seyfert 1 galaxies

Date: 2018-11-01
Speaker: Dr. C.S. Stalin (IIA, Bangaluru)
Venue: Thaltej Seminar Room (113/114)

Abstract

Not available

The Largest Particle Accelerators of the Universe

Date: 2018-09-13
Speaker: Dr. Kamlesh Rajpurohit (TLS Observatory, Germany)

Abstract

Galaxy clusters are the most massive, gravitationally bound systems in the Universe and are unique laboratories to probe the physics of particle acceleration and magnetic field properties. Galaxy clusters are formed by accretion of gas and mergers with other clusters and galaxy groups. During cluster formation processes, some of the energy released is channeled into merger driven shocks and turbulences, leading eventually to the acceleration of particles to relativistic energies. However, the underlying particle acceleration mechanism is still being debated. In this talk, I will discuss how merging galaxy clusters can act as giant cosmological particle accelerators and about the magnetization of the intra-cluster medium.

NICSPol, A Near Infrared polarimeter for the 1.2m telescope at Mount Abu Infrared Observatory

Date: 2018-08-16
Speaker: Ms. Aarthy E. (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

NICSPol is a near infrared imaging polarimeter developed for the Near Infrared Camera and Spectrograph (NICS), one of the back end instruments of the 1.2 m Cassegrain telescope at the Mount Abu Infrared Observatory (MIRO), India. The polarimeter consists of a rotating wire grid polarizer which is mounted between the telescope optics and NICS. NICSPol is a general purpose instrument which could be used to study various astrophysical sources like star forming regions, AGNs, Pulsars, XRBs, GRBs, Supernovae et cetera. The observations are carried out for sources by rotating the polarimeter at multiple angles to determine the Stokes parameters, which are then converted into the polarization fraction and polarization angle. In this talk I would brief about the instrument, data acquiring and analysis procedure. The performance of NICSPol is verified with polarimetric standards along with which a preliminary science result will also be discussed.

Spectral and Timing studies of Be X-ray binary pulsar 2S 1417-624

Date: 2018-08-13
Speaker: Shivangi Gupta
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Be/X-ray binaries are among the brightest transient X-ray sources in our Galaxy. The compact object in these systems (which is a mostly a neutron star), revolves in a wide eccentric orbit around massive Be spectral type optical companion. These sources show periodic or Type-I X-ray outbursts, usually associated with periastron passage of binary. It is believed that neutron star accretes matter from a circumstellar disk around the Be star and powers such a strong outbursts. Another kind of X-ray enhancements, Type-II outbursts are also displayed by these systems, which are quite rare and independent of the orbital phase of the binary system. These rare events provide a unique opportunity to probe the emission geometry of these binary system. In this talk I will present results obtained from the spectral and timing studies of such a source, 2S 1417-624 during a Type-II outburst in 2009, using RXTE observations.

An Infrared study of dark cloud : LDN1340

Date: 2018-08-09
Speaker: Ms. Archita Rai (PRL, Ahmedabad)

Abstract

Lynds dark nebulae (LDN) are dark patches seen against the background stars of the Milky Way. They are thought to be the birth places for stars. A catalog of these dark nebulae was made in 1960 by Beverly Turner Lynds, who visually inspected large areas of the Milky Way on the Palomar-Schmidt Survey Plates (POSS). As these regions have large opacity (range ~ 1 to 6) & extinction caused by high density and interstellar dust, the stars embedded in the dark clouds may not be visible in the optical band. To study them, infrared wavelength are expected to provide us with more penetration depth to study the core of the clouds. Hence more stars can be detected and the phenomena like polarisation and extinction which depend upon the distribution of the interstellar dust and the interstellar magnetic field can be studied in detail. In my talk, I will give a brief introduction to one of the Lynds clouds (LDN1340) and discuss its behaviour and properties. I will be discussing the observational aspects of using NICSPOL at the Mt. Abu Infrared Observatory along with other archival photometric data for this study.

Cold gas in high redshift galaxies

Date: 2018-07-16
Speaker: Dr. J.N.H.S. Aditya
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Cold HI gas at high redshifts can be probed using HI 21 cm absorption against background radio-loud Active Galactic Nuclie (AGN). The distribution and kinematics of this gas, present in the vicinities of the AGN, can be studied using this technique. A large number of surveys in last 50 years have reported nearly 140 detections of such `associated' HI 21 cm absorbers. However, most of these studies are limited only to low redshifts (z < 1). Using GMRT we have carried out a search for associated HI 21 cm absorption in a large and uniformly selected sample of 76 flat-spectrum radio AGN that inclues 50 sources at z > 1. For the first time, we find a statistically significant evidence (~3-sigma) that the strength of associated HI 21 cm absorption in the high-z sub-sample is lower than that of the low-z sub-sample. Also, we detect four new HI 21 cm absorbers at z > 1, increasing the total number of known absorbers at z > 1 to eight. The possible causes for the low absorption strength at higher redshifts can be : (1) redshift evolution in the AGN environments, (2) high UV and/or radio luminosity of high-z AGN, and (3) low gas covering factor in high-z AGN. These plausibilities are further investigated in Giga-Hertz Peaked Spectrum (GPS), High-Frequency Peaker (HFP) and other AGN with compact radio-jets. I shall present the details of our surveys and the studies of individual cases of newly-detected HI 21 cm absorbing systems.

Accretion on to LMXBs: A multi-wavelength approach

Date: 2018-07-06
Speaker: Mr. Sandeep Rout (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Most Low mass X-ray binaries (LMXBs) show a transient behavior wherein they spend most of their life in quiescence and occasionally undergo outbursts due to instability in the accretion disk. These sources pose a unique opportunity to study the properties of the entire accretion disk especially of the relatively cool outer disk as the contribution of the companion star is negligible. One such soft X-ray transient GRS 1716-249 had been in an outburst during December 2016 and was observed in optical and infrared bands from Mt Abu Infrared Observatory. Towards the end of the outburst period, it was also observed by AstroSat as a ToO source. Combining the data obtained from Optical/IR and X-rays (3.0 - 100 keV) a multiwavelength analysis is attempted which promises to provide better constraints on black hole mass, disk outer radius etc. and help in expanding our understanding of various energetic processes occurring at the disk. In the seminar, I will discuss the general theory of accretion disks followed by the work carried on so far on the above object.

X-ray Superflares on Solar-type Stars

Date: 2018-05-03
Speaker: Dr. Subhajeet Karmakar

Abstract

Flares on the Sun and the stars are generally interpreted as a rapid and transient release of magnetic energy in coronal layers driven by reconnection processes, associated with electromagnetic radiation from radio waves to γ-rays. The typical total energy of a solar flare ranges from 10^29 to 10^32 erg, whereas Superflares are defined as the flares with the total energies of 10^33–10^38 erg. Although there are few hundreds of Superflares have been already discovered in optical waveband, the discoveries of X-ray superflares are still rare events (only nine have been discovered). The study of superflares on solar-type stars provides us important constraints of the upper end of the dynamical behavior of stellar corona as well as it helps us to probe the possibility of superflares on the Sun. In this seminar, I will talk about the analysis of three X-ray superflares. The first two superflares occurred on a fast-rotating (period of 1.56 day) main-sequence star CC Eri, and the third one occurred on an evolved 3.966-day period RS CVn type eclipsing binary system SZ Psc. In case of the superflares from CC Eri, it has been found that the flares decay faster in the hard X-ray band than in the soft X-ray band. Both flares are highly energetic with respective peak X-ray luminosities of ∼10^{32.2} and ∼ 10^{31.8} erg s^{−1} in 0.3-50 keV energy band. The time-resolved spectral analysis during the flares shows the variation in the coronal temperature, emission measure, and abundances. The observed peak temperatures in these two flares are found to be 174 and 128 MK. Using the hydrodynamic loop modeling, we derive loop lengths for both the flares as 1.2+/-0.1 x 10^{10} cm and 2.2+/-0.6x10^{10} cm, respectively. The Fe Kα emission at 6.4 keV is also detected in the X-ray spectra and we model the Kα emission feature as fluorescence from the hot flare source irradiating the photospheric iron. These superflares are the brightest, hottest, and shortest in duration observed thus far on CC Eri. On the other hand, the superflare on SZ Psc lasted very long duration for more than 100 ks, which is the longest duration X-ray flare ever observed as the best of our knowledge. Spectral analysis indicates a presence of one temperature corona, which represents the flare temperature. The temperature is one of the highest observed spectroscopically with a peak at 258 MK, which is ∼10 times more than the observed minimum value. The length of the flaring plasma was derived to be 7.3 x 10^{11} cm. The longer coronal loop and high level of magnetic activity at the coronal height for the RS CVn type binary star is probably due to the presence of extended convection zone of the sub-giant and the high orbital velocity.

Astronomy & Astrophysics Division Seminar

Date: 2018-05-01
Speaker: Mr. Kumar Venkataramani (PRL, Ahmedabad)

Abstract

Comets are cold, icy bodies in the Solar system that were formed in the early solar nebula. They are considered to be the signature bodies for investigating the formation of the Solar system. As a comet nucleus makes its journey towards the Sun, the ices start sublimating, giving rise to a mixture of gas and dust that forms the coma. Cometary molecular emissions are well known and have been studied since a long time. A typical optical spectrum of a comet with well developed coma shows molecular emissions dominated by carbon chain molecules like C2 and C3. NH2 and CN are two other species which show prominent emission lines in the optical spectrum. In general, these molecular emissions start appearing sequentially when the comet comes closer than 3 AU to the Sun. The most likely emission to appear first is that of CN molecule at around 3 AU, followed by the rest of the emissions. There are very few instances, when these pristine ice balls show a distinct behavior as compared to the regular oort cloud and jupiter family comets. Such a behaior is a consequence of its distinctive processing at the location of its formation in the early solar nebula. The spectroscopic observations of the comet C/2016 R2 have revealed its unusual behavior in the last few months. The results from the study of this distinct visitor to the inner solar system would be discussed in this talk.

Enigma of Be/X-ray binary pulsars near the critical luminosity

Date: 2018-04-24
Speaker: Mr. Prahlad Epili
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Be/X-ray binaries (BeXBs) form a major sub-group of High mass X-ray binaries. In these systems, a neutron star (also a pulsar) co-rotates with a normal O or B-type star, and accretes a copious amount of matter from an extended disk. Accretion of the matter into the enormous gravitational field of neutron star gives rise to X-ray outbursts. Most of the high energy emission originates from a tiny region on the surface of the pulsar, known as accretion column. For the present study, two BeXB pulsars were probed using RXTE, Suzaku and NuSTAR observations to understand the column emission, its geometry, and spectral characteristics as a function of mass accretion rate. We have also seen that the luminosity dependence of pulse profiles and spectral parameters could explain the existence of different accretion regimes (above and below a specific or critical luminosity) in these sources. It also helped us in estimating the magnetic field of neutron star that was remained ambiguous due to non-detection of cyclotron line(s) in them. The findings from our work will be discussed in detail in this talk.

Heating in solar active regions and the importance of X-ray observations

Date: 2018-03-22
Speaker: Giulio Del Zanna
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

I review a few key observational aspects about how we constrain the heating theories in the cores of active regions. Most of our recent knowledge comes from EUV/UV observations, where very little information on the hot plasma, formed above 3 MK, can be obtained. The soft X-rays around 100 Angstroms are a promising candidate in the future, although important constraints will soon be obtained from X-ray spectroscopy around 10 Angstroms, where measurements of chemical abundances are also possible.

Photohadronic process: a novel mechanism to explain the multi-TeV flaring from high energy blazars

Date: 2018-02-15
Speaker: Dr. Sarira Sahu
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Many high-energy blazars (HBLs) are observed flaring in multi-TeV gamma-rays and the emission mechanism is not understood well. The traditional leptonic model has difficulties to explain these emissions. These high-energy gamma-rays get attenuated due to extragalactic background light. So we can't see very-high energy gamma-rays from far-off sources. We have developed a photohadronic model to explain these flarings and are able to explain most of them very well. However, our model prediction depends on the spectral energy distribution (SED) of the synchrotron self-Compton (SSC) photons which is so far not been measured but fitted using leptonic processes. In this talk I shall discuss about few flaring blazars and their interpretation using our model. Also, I shall comment on the importance of observing the photon flux in SSC band by satellite-borne experiments to understand the emission mechanism in blazar jet. This observation will also shed more light on the production of very high energy neutrinos by IceCube and also dark matter in the mass range (few keVs to about 100 MeV).

Probing the central engine in Seyfert 1 galaxies

Date: 2018-02-08
Speaker: Dr. Main Pal
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Understanding the nature of the unresolved central engine has reached to a great level in Seyfert 1 galaxies. Though we do not understand actual character of the central engine, we study these sources through timing as well as spectroscopic tools. Using these tools, we investigate them in the multi-wavelength range such as the UV/Optical as well as the X-ray bands. One of the major interest is to understand the UV/Optical emission variability and its connection to the X-ray emission. In this direction, I will be discussing about the recent results on a narrow line Seyfert 1 galaxy NGC 4593 and a broad line Seyfert 1 galaxy 1H 0419-577. In the NGC 4593, I have made a detailed study on strongly correlated UV/Optical/X-ray emission and I found that the real disk may not be as the predicted one Shakura-Sunyaev standard disk. In the 1H 0419-577, the marginally correlated UV/Optical/X-ray emission show a complex origin of the UV/Optical variability possibly a mixture of reprocessing and the changes in the geometry of X-ray emitting corona. I will also present some ongoing work on a NLS1 Mrk 766. This shows moderate harder when brighter state and soft emission is leading to the hard emission.

Investigation of Sun-like G Stars and Their Exoplanets

Date: 2018-01-29
Speaker: Shashanka R. Gurumath

Abstract

Humans' quest is to understand how the universe is originated and has been evolved; how the stars, planets and finally, life is emerged on the Earth? How the Sun and Solar system are formed? Although it is too naive, nebular hypothesis of solar system formation doesn't explains many of these intriguing observations of solar system, especially the angular momentum problem and architecture of solar system, etc. In order to find clues for some of these problems, by employing exoplanetary data, we have carried out a research on understanding the influence of dynamics, chemical abundance and magnetic field structure of Sun-like G stars on the formation and orbital migration of their exoplanets. During this talk, I will also discuss some of the results (possible solution for the faint young sun paradox, missing mass in the vicinity of the sun, etc..,) that are emerged from this study

X-ray and radio views of galaxy clusters

Date: 2018-01-11
Speaker: Dr. Viral Parekh
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Galaxy clusters are the most massive bound and quasi-relaxed objects in the Universe. In recent years multiwavelength observations have shown the presence of substructures related to merging events in a high fraction of galaxy clusters. Radio interferometry observations have shown the existence of diffuse large scale emission from the intracluster medium (ICM), suggesting that non-thermal components, magnetic fields and relativistic particles, are mixed with the hot ICM. Currently diffuse radio sources are grouped in two classes. One class is known as radio halos located at the cluster centre and other class is radio relics situated at the cluster periphery. In this seminar, I will talk about our recent findings of new clusters which show varieties of X-ray features, and its corresponding radio observations. I will also talk about the GLEAM (GaLactic and Extragalactic All-sky MWA survey) survey which helps us to investigate new galaxy clusters. GLEAM is a survey of the entire radio sky south of declination +25 deg at frequencies between 72 and 231 MHz made with the MWA (Murchison Widefield Array), the low-frequency Square Kilometre Array (SKA1 LOW) precursor located in Western Australia. Finally I will present some of our results obtained using the upgraded GMRT.

The Study of blazars with AstroSat and prospectives in multi-messenger era

Date: 2018-01-04
Speaker: Dr. Sunil Chandra
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The blazars are subclass of Active Galactic Nuclei (AGN), having a bipolar jet of ultra-relativistic plasma oriented very close of our line of sight. Seen at very close angles, the emission from blazars is mostly dominated by the jet. Therefore, the blazars provide a unique sample to understand the acceleration and collimation mechanisms, taking place very close to the central engine. It also enables us the diagnostics of magnetic field, jet morphologies and particle energy distributions. We have been monitoring a sample of bright blazars, utilizing the unique capacity of AstroSat, i.e., the strictly simultaneous coverage from opt/UV to hard X-rays. These observations are mostly co-ordinated with optical/IR observations by ground based observatories, namely MIRO and HCT. During this seminar I aim to discuss a number of interesting results obtained from the studies performed over the course of last two years. The blazar's jet can also be treated as very efficient particle accelerator. The neutrino events are expected to coincide with the flares in blazars. The recent observations by ICECUBE and HAWC have shown some positive hints to this hypothesis, however, still no firm confirmation is obtained. I will also try to discuss the possible contributions of multi-messenger approach to understand the blazar's jet.

2017

Gearing up for India's Large Optical-NIR Telescope

Date: 2017-12-06
Speaker: Prof. Padmakar Parihar
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Recently install 3.6m DOT telescope and forthcoming TMT would not be enough to cater the need of growing Indian astronomical community. Access to 10 - 12m size optical-NIR telescope equipped with state of the art back-end instruments can bridge the gap between DOT and TMT. A telescope of this size is only possible when primary mirror is made of smaller mirror segments. The participation into TMT project has given us an opportunity to adopt some the key technologies linked with segmented mirror telescope(SMT) and make use of resources being developed for this mega project within India. In my talk I will present the possible sites for the proposed telescope, preliminary opto-mechanical design and some R&D effort happening in IIA to explore SMT technology by developing a prototype segmented mirror telescope.

FRB121102 : First detection across 4 - 8 GHz, spectral and polarization properties

Date: 2017-11-16
Speaker: Dr. Vishal Gajjar
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Fast Radio Bursts (FRBs) are some of the most energetic and enigmatic events in the Universe. The origin of these sources is among the most challenging questions of modern-day astrophysics. Thus, it is imperative to understand the properties of these bursts across a range of radio frequencies. Among the known FRBs, FRB121102 is the only source known to show repeated bursts [Spitler et al., Nature, 531, 7593 202-205, 2016], which can allow a detailed investigation of various origin models. In August 2017, we initiated a campaign observing FRB 121102 using the Breakthrough Listen Digital Backend with the C-band receiver at the Robert C. Byrd Green Bank Telescope (GBT). We recorded baseband voltage data across 5.4 GHz of bandwidth, completely covering the C-band receiver's nominal 4-8 GHz band [MacMahon et al. arXiv:1707.06024v2]. The recorded data were searched for dispersed pulses consistent with the known dispersion measure of FRB 121102 (557 pc/cm^3) using high-speed GPU software tools. We detected 21 bursts above our detection threshold of 6 sigmas in the first 60-minutes, out of which 18 occurred in the first 30-minutes only. To our knowledge, this is the highest event rate seen for FRB121102 at any observing frequency. These observations are the highest frequency and widest bandwidth detection of bursts from FRB 121102 (or any other FRB) obtained to-date. We note that individual bursts show marked changes in spectral extent ranging from hundreds of MHz to several GHz. We have used high frequency dynamic spectra of these bursts to estimate the characteristic scintillation bandwidth and correlation time-scale. We also found distinctive temporal structures, separated by a few milliseconds, in three of the strongest bursts, with each sub-structure exhibiting varied spectral features. We will discuss our findings and how these detections of FRB 121102 around 8 GHz opens up a new regime in scrutinizing various origin models. We will also highlight the unique capabilities of the Breakthrough Listen instrument at the GBT which allowed such sensitive and detailed observations.

A comprehensive study of open clusters

Date: 2017-11-02
Speaker: Dr. Devendra Bisht
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Our main aim is to study the astrophysical behaviour of poorly studied open star cluster NGC 5617 towards, longitude (l) = -45 deg in the Galactic plane using PPMXL, 2MASS, VVV survey, WISE and GLIMPSE data. We also have studied the mass function and dynamical state of the clusters Berkeley 24 and Czernik 27 using the optical CCD data for the first time. Mass function is derived by considering the corrections of data incompleteness and field star contamination. The mass function in the outer region of clusters are found to be steeper than in the inner region, indicating the presence of mass-segregation effect.

Deep learning in astronomical research

Date: 2017-10-12
Speaker: Mr. Vaibhav Dixit
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Artificial Intelligence (AI) is the capability of the machines to perform a task intelligently like humans. Deep learning is a subset of AI which takes inspiration from model of human brain. It is based on neural networks with several layers of nodes. Deep learning encourages computational models that are built on multiple layers to learn representations of data with multiple levels of abstraction. It has surpassed the human performance in several trivial tasks like image recognition and segmentation as well as found applications in various commercial activities like self-driving cars, stock market prediction, cancerous tissue identification and production line performance etc. Given the sheer magnitude of projects undertaken by various organizations, we would end up having Petabytes of data, and no manpower to go through all of them manually. Under such a scenario, we would need intelligent systems which can devour large data. Higher dimensionality of the data, no prior knowledge of the data generation process, low signal to noise ratio and missing information are some other limiting factors in analysis done by humans. However deep learning overcomes these challenges and can open new avenues for scientific research. Deep learning is going to change the way we do science. The talk will focus on some areas of applications of deep learning in astronomy.

Long term X-ray variability of Be/X-ray binaries : a case study of EXO 2030+37

Date: 2017-10-03
Speaker: Prahlad Epili
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

In X-ray astronomy, the study of X-ray binaries shows interesting features of the stellar X-ray sources. Among these the spectacular variability seen in High mass X-ray binaries (HMXBs) at different time scales can be studied to understand the nature of the two stellar components, the physics of matter at extreme physical conditions, the ‘accretion’ process governing the mass transfer to the compact object etc. Majority of these HMXBs are bright and transient in nature. One such class of bright X-ray transients are Be/X-ray binaries. In theses sources, an abrupt accretion of huge amount of matter from the normal star onto the neutron star is seen with X-ray outbursts, i.e an enhancement in the X-ray emission. Study of these X-ray outbursts (some are periodic and some are aperiodic) forms an important basis to further our understanding of these sources. EXO 2030+375 is a transient Be/X-ray binary system. It has been caught in X-ray outbursts numerous times during 19962011 in the RXTE era. We have studied the decade long X ray observations of this binary to understand its long term X-ray variability. The details of the study will be presented.

Recent Developments in Electronic Subsystems of PARAS

Date: 2017-09-07
Speaker: Mr. J. S. S. V. Prasad
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

PRL Advanced Radial Velocity Abu Sky Search (PARAS) is a high-resolution Spectrograph which uses radial velocity technique for detection of exoplanets. In order to reach an accuracy of Doppler shift measurements of a few meters-per-second, effective temperature regulations of the stellar spectrograph is very important. A change of 1°C in the spectrograph temperature can produce instrument shift of 100m/s due to change in refractive index in Lenses and other optical components. Hence temperature variation during the observation should be within 0.01°C to 0.005°C to get a system stability of 1m/s or better. In this talk, I will discuss about the Temperature controller of PARAS along with some recent developments in other electronics sub systems.

In situ 1.2m telescope mirror aluminizing facility at Mount Abu

Date: 2017-08-31
Speaker: Mr. S. N. Mathur
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Mirrors used in astronomical telescopes are one of the primary components of their optical system. Aluminum coated mirrors used in telescopes are exposed to the local environment, and as a result, mirror surface degrades over the time. The degradation leads to decrease in reflectivity and increase in light scattering and infrared emissivity. In order to achieve better reflectivity, mirrors are routinely re-coated. The aluminization of mirrors is a delicate process as it involves the handling of big optical system. Therefore, it is always better to have an in-situ mirror coating plant. For our 1.2m telescope at Mt. Abu, we have an in situ mirror aluminizing facility. In this talk I shall describe the aluminization process and the facility.

Near-IR Imager Spectro Polarimeter (NISP) electronics Development

Date: 2017-08-28
Speaker: Ms. Deekshya Roy Sarkar (PRL, Ahmedabad)
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Near-IR Imager Spectro Polarimeter (NISP) with H2RG detectors is being developed as a back-end instrument for the upcoming 2.5m telescope at Mt Abu. In this talk, I shall discuss about the two approaches (discrete and ASIC based) that we have adopted for developing the front end electronics of the instrument. I shall also talk about the advantages and performance characteristics, and the work done so far.

Neutrino Astronomy with IceCube Neutrino Detector

Date: 2017-08-17
Speaker: Dr. Debanjan Bose
Venue: Room - 113/114, Thaltej Campus

Abstract

IceCube is a cubic-kilometer neutrino observatory buried deep in the ice sheet at the geographic South Pole. A total of 5160 Digital Optical Modules (DOMs) are deployed on 86 strings forming a three dimensional detector array. Over the past decade, South Pole has emerged as a leading site for neutrino astronomy, particle astrophysics and neutrino physics. IceCube's discovery of a diffuse flux of astrophysical neutrinos started a new era of neutrino astronomy. In this talk I will describe IceCube neutrino detector and some of it's results. Also, I will give an overview of plans to upgrade IceCube.

Radioactive sources of APXS on-board Chandrayaan-2: Implications

Date: 2017-08-10
Speaker: Mr. Mithun N. P. S.
Venue: Room - 113/114, Thaltej Campus

Abstract

Alpha Particle X-ray Spectrometer (APXS) experiment on-board Chandrayaan-2 rover employs the techniques of X-ray Fluorescence (XRF) and Particle Induced X-ray Emission (PIXE) to measure the elemental abundances of Lunar surface close to the landing site. For this purpose, APXS carries six Cm-244 sources of total activity of 30 mCi, which emits X-rays as well as alpha particles. Usage of these high activity sources calls for certain special considerations in the design of the instrument, assembly, characterization and calibration, transportation and spacecraft level integration tests to ensure the safety of people around and better performance of the instrument. In this talk, we will discuss the aspects of radiation safety, radiation dose calculations, measurements, and limits, transportation regulations etc in the context of APXS.

Mechanical design and realisation of Chandrayaan 2 Payloads

Date: 2017-07-28
Speaker: Mr. Hiteshkumar Lavjibhai Adalja
Venue: Room - 113/114, Thaltej Campus

Abstract

Solar X-ray Monitor (XSM) on-board Orbiter to provide the real time solar X-ray flux incident on the lunar surface and Alpha Particle X-ray Spectrometer (APXS) on-board Orbiter to measure the elemental composition of lunar soil and rocks near the landing site are being developed at PRL for upcoming CHANDRAYAAN-2 mission. In this presentation I will discuss mechanical design and development activities of these two payloads.

Sensing the space rocks – the optical spectrum of asteroids

Date: 2017-07-13
Speaker: Mr. Kumar Venkataramani, PRL, Ahmedabad
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The asteroids revolving around the Sun today may have been a part of some major planetary body, a failed planet or just broken bits of rock ever since their formation in the proto-planetary disk. Today, the major chunk of these space rocks are found clustered around at different locations in the solar system, gravitationally bound by the major planets. The reflectance spectra and the geometric albedo of these asteroids depend on their surface composition, the macroscopic roughness of surface, opposition surge brightness at low phase angles and the overall shape of the asteroid. Based on their reflectance slopes, albedos and absorption bands, these asteroids have been put into various taxonomic classes and sub-classes. Near-Earth flyby's of some of these asteroids give us good opportunity to study them with a large phase angle coverage. Two such Near-Earth flyby's were spectroscopically followed with the optical low resolution spectrograph LISA mounted on the 0.5m and 1.2m telescopes at the Mount Abu Infra-red Observatory(MIRO). One of them being the recent close shave of the potentially hazardous asteroid 2014JO25 in April 2017. Time and phase resolved spectra of this asteroid gave very interesting clues on its uniqueness. Apart from this, we have also spectroscopically followed many of the main-belt asteroids of different taxonomic classes at different times. This talk will give a brief overview of the different classes of asteroids and some of the key results from their optical spectra.

Understanding the enigma of accretion powered X-ray binary pulsars

Date: 2017-07-12
Speaker: Dr. Gaurava Jaiswal, PRL, Ahmedabad
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Be/X-ray binaries are among the brightest transient sources in the Galaxy. They mostly harbor neutron star as a compact object, revolving in a wide eccentric orbit around massive Be spectral type optical companion. These sources show periodic or Type-I X-ray outbursts, usually associated with periastron passage of binary. It is believed that neutron star accretes matter from a circumstellar disk bound to the Be star and powers such strong outbursts. We have studied Be/X-ray binary pulsars such as Cep X-4, GX 304-1 and SMC X-2 during Type-I X-ray outbursts for understanding the characteristics of emission geometry, accretion physics, evolution of spectral continuum, cyclotron lines, and magnetic field mapping around these exotic objects, by mainly focusing on timing and spectral aspects using Suzaku and NuSTAR observations. The results obtained from these studies and some other accretion powered X-ray pulsars will be discussed in this talk.

Timing and spectral properties of accretion powered X-ray pulsars

Date: 2017-06-27
Speaker: Ms. Shivangi Gupta, PRL, Ahmedabad
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Accreting X-ray pulsars are the brightest objects in the X-ray sky. They are usually found in an X-ray binary system in which a neutron star accretes matter from an optical companion via Roche robe overflow or through the strong stellar winds emanating from its companion. During the accretion process, the matter from donor interacts with the pulsar’s strong magnetic field and forced to follow the field lines to magnetized poles of the neutron star. This results in the formation of a column like structure on the top of the neutron star, which acts as the main source of pulsating X-ray emissions. The basic structure of the emitting region, accretion physics, and tool to probe the magnetic field can be studied by understanding the timing and spectral properties of pulsars. In this talk, I will discuss the properties of such interesting sources along with a curious case study of X-ray binary pulsar GX 1+4.

Cold Gas at High Redshift

Date: 2017-06-21
Speaker: Dr. Nissim Kanekar
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The gas and dynamical masses of damped Lyman-alpha absorbers (DLAs), a population of absorption-selected galaxies, have been open questions in the field of galaxy evolution for more than three decades. This talk will describe new results from Arecibo and ALMA searches for HI 21cm, CO and CII-158 micron searches emission from a sample of DLAs at, respectively, low (z < 0.1), intermediate (z~0.7) and high (z~4) redshifts. Our HI 21cm observations of the DLAs at z<0.1 yield normal gas masses, <~ 5 x 10^9 solar masses, but very high ratio of gas mass to stellar mass, ~ 5-100, far higher than in normal galaxies. For the absorbers at intermediate redshifts, we obtain large molecular gas masses in the six systems with CO detections, despite their low optical star formation rates. We also obtain high star formation rates (based on dust continuum emission) and high CII-158 micron line luminosities for the DLA host galaxies at z~4. For the CO and CII-158 micron detections, the impact parameters of the host galaxies are high, 15-45 kpc, far larger than expected based on earlier studies. These are the first CO and CII-158 micron detections in DLA hosts, providing a new window on physical conditions in absorption-selected galaxies, and yielding a new tool to identify DLA host galaxies at high redshifts.

The magic of flares in Gamma-ray bright blazars

Date: 2017-05-11
Speaker: Ms. Navpreet Kaur
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The advancement of Gamma-ray detection techniques (since the launch of Fermi in 2008-post CGRO) has helped our understanding of one of the most powerful Cosmic accelerators in the Universe, called blazars. Blazars prove to be one of the promising candidates in exploring the enigmatic nature of AGN, during their flaring (or outburst) in gamma-rays and all the way down to radio frequencies. Since their emission is detectable across the whole spectrum, their variability properties across all timescales, from years to few minutes, shed light on their emission processes inside the jet. Flaring phases in these sources are helpful in understanding their acceleration/cooling mechanisms and their connection across all other frequencies. In this talk, I will discuss about flaring patterns in gamma-ray bright blazars, their role in the understanding till date with a few examples from our study.

Automatic Analysis Tools for Stellar Parametrization and classification of cool stars

Date: 2017-05-05
Speaker: Dr. Kaushal Sharma
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

With the ongoing/upcoming large ground and space-based surveys, such as Kepler mission, Hubble Space Telescope, AstroSat, SDSS, Gaia, JWST etc., large astronomical databases have become accessible in the real-time. But, growth in the data reduction and analysis sector does not match up well with the corresponding growth in data collection domain. However, automatic stellar parameter (Teff, logg and [Fe/H]) determination from low-resolution spectroscopy has seen some advancement in the past two decades. Automated algorithms for parameterization (or spectral classification) can be broadly put into two categories: Minimum distance methods (MDMs) and Non-linear regression methods. An example of each of these techniques, Full spectrum fitting and Artifical Neural Networking (ANN), would be discussed. Full spectrum fitting has been applied and improved for cool stars which are the most probable targets for the exoplanet studies. Moreover, these are the crucial player for galactic kinematics and chemical enrichment analysis. Therefore, their accurate parameter determination becomes of vital importance. Improved parameters of cool stars using full spectrum fitting technique will be presented.

Amplitude of solar wind density turbulence from 10 - 45 Rs

Date: 2017-05-04
Speaker: Dr. Madhusudan Ingale
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Turbulent fluctuations in the solar wind leads to wide variety of phenomena. Amplitude of solar wind density turbulence ($C_N^2$) and density modulation index ($\epsilon_N$) are two important parameters often invoked to interpret various observations as well as transport properties of the solar wind. In this work we report on C_N^2 and \epsilon_N in the solar wind between 10 and 45 R_{\odot}. We derive these quantities using general structure function, observationally constrained by the occultation observations of the crab nebula made in 2011 and 2013 and similar observations published earlier. Our work yield a comprehensive picture of, a) the manner in which C_N^2 and \epsilon_N vary with heliocentric distance in the solar wind and b) Solar cycle dependence of these quantities.

Quantifying the accuracy and precision of difference-smoothing algorithm for time delay measurement using simulated light curves from TDC1

Date: 2017-04-27
Speaker: Dr. S. Rathna Kumar
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Measurement of time delays between the images of gravitationally lensed quasars has been demonstrated to be a competitive method to constrain cosmological parameters. During the next decade, the Large Synoptic Survey Telescope (LSST) is expected to discover and monitor on the order of thousand lensed quasars. Hence it is of interest to develop time delay measurement algorithms that are not only fast, but also accurate and precise. In this talk, I shall describe a recent effort to rigorously test the difference-smoothing algorithm on a large sample of simulated light curves from the first edition of Strong Lens Time Delay Challenge (TDC1) and the refinements that were required to make it competitive with the best performing algorithms of TDC1. I shall also briefly touch on the upcoming TDC2, which is expected to address the problem of reliably measuring time delay from a combination of multi-filter light curves as expected from LSST.

Xray/UV/Optical variability in Seyfert 1 galaxies

Date: 2017-04-20
Speaker: Dr. Main Pal
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Active galactic nuclei (AGN) are the most luminous, long-lived and highly variable sources in the Universe. These objects emit almost over entire electromagnetic spectrum. In case of Seyfert type 1 AGN, the UV/optical to X-ray spectrum consists of complex spectral components such as powerlaw, soft X-ray excess below ~2 keV, iron- K lines near 6 keV, Compton hump in 10-50 keV and big blue bump (BBB) in the UV/optical bands. These components can be used to probe the unresolved central engine and various physical processes in the vicinity of the supermassive blackhole. The variability nature of these features in the X-ray to UV/optical bands are less well understood. I will discuss these components and their relationship through spectral and timing variability study in detail in Seyfert 1 galaxies such as 1H 0419-577, II Zw 177 and Fairall 9.

GRB Polarization using AstroSat CZTI

Date: 2017-03-02
Speaker: Ms. Aarthy E
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Gamma Ray Bursts are the most powerful explosions observed so far in the universe since the Big Bang. Since the time it was discovered, over the past 4 decades the field of GRBs has grown tremendously with the advent of several missions like CGRO/BATSE, Swift, Fermi etc and other ground based observatories. Although, there are still unanswered questions related to the prompt emission (initial burst of gamma rays), the emission mechanism behind the inner most region of the relativistic jets. And measuring GRB Polarization could take this one step further. CZTI onboard AstroSat has an added advantage of being a GRB monitor and has capabilities to measure polarization at energies > 100 keV. I would brief about the current understanding of GRBs, how CZTI works as a polarimeter, and also would present the polarization results we have obtained for few GRBs using CZTI.

Bayesian Inference : An Approach in Astronomy

Date: 2017-02-16
Speaker: Ms. Archita Rai
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Statistical methods of data analysis have been used in science since centuries, with slight modifications which can be traced to the same school of thought – the Bayesian approach (1763). In the astronomical domain, usage of Bayesian statistics has started in the last few decades and since then it is considered to be the most referred one by researchers in this field. The Bayesian method is an application of the Bayes' theorem. Bayes' theorem provides the conditional probability of the happening of an event given that another event has taken place. When applied to models used to fit a particular data set, it helps in getting the model parameters which best fits the observed data. Bayesian statistics, hence, deals with the data that were actually observed. With a brief overview of probability, the talk will try to explain the 'Bayesian recipe' with a detailed look on the different probability distributions from prior to posterior involved in it. The Bayesian approach towards data sets and model selection will also be discussed with its application in astronomy. The Bayesian inference method exemplifies the statement : 'It is vain to do with more what can be done with less'.

Bridging Macro and Micro Physical Scales in Astrophysical Jets using Numerical Simulations.

Date: 2017-01-19
Speaker: Dr. Bhargav Vaidya
Venue: Room - 113/114, Thaltej Campus

Abstract

Jets and Outflows is a universal phenomenon that marks the onset of active accretion process in various astrophysical objects. With an aim to connect macro-physical models and create a synthetic observatory for such astrophysical jets, numerical simulations should incorporate effects due to microscopic processes. In this talk, I will focus on two such microscopic processes, viz. thermo-chemical evolution and particle acceleration, that have been implemented in the PLUTO code. The thermo-chemical module accounts for combining fluid dynamical models with chemical evolution in presence of consistent Equation of State (EoS). I will present results on applying this module to study molecular outflows in young stellar objects. Further, in order to study physics of particle acceleration and multi-wavelength emission from AGN jets, I will present an implementation of a fully parallel, hybrid framework that allows the evolution of Lagrangian particles coupled to relativistic MHD flows. A spectral evolution study in conjunction with propagation of particles through regions of varying magnetic field and associated radiative processes like synchrotron emission, Inverse Compton provides a consistent picture of particle energetics.

2016

The Inner Gaseous Disks of Herbig Be Stars

Date: 2016-12-15
Speaker: Dr Parshati Patel
Venue: Room-113/114, Thaltej Campus

Abstract

Herbig Ae/Be (HAeBe) stars are massive pre-main sequence stars known to have disks and envelopes around them. These disks are inherited by the star from the star formation phase and can generally be divided into two regions: gaseous and dusty. The disk region close to the star experiences intense radiation, creating high-temperatures and leading to sublimation of the dust and an entirely gaseous region. This part of the disk, also known as dust-free zone, is poorly understood. This inner gaseous disk could be very similar to the disks around other hot stars such as Classical Be stars, whose disks are reasonably well understood. Using the circumstellar disk codes, BEDISK and BERAY, to model observed optical and near-infrared spectra from the ESPaDOnS instrument on Canada-France-Hawaii Telescope, we attempt to constrain the physical properties of the inner gaseous region. The photoionizing radiation of the central star is assumed to be the sole source of input energy for the disk. I will present the results from our study of four early-type Herbig Be stars

Millimeter wave astronomy at the Institute of Applied Physics RAS: Instrumentation development and scientific results

Date: 2016-11-10
Speaker: Prof. Igor Zinchenko
Venue: Room - 113, Thaltej Campus

Abstract

In this review talk I will describe first the history and the current status of the developments of astronomical instrumentation at the Institute of Applied Physics of the Russian Academy of Sciences. They include low noise millimeter and submillimeter wave receivers, cryogenic systems for astronomy and other applications, precise laboratory spectroscopy, etc. In the second part I will present some results of our astronomical research, based mainly on millimeter and submillimeter observations of molecular spectral lines and dust continuum: surveys of high mass star forming regions, detailed investigations of several selected objects, observations of high-redshift objects, etc.

Structure and the chemical composition of the W40 dense core

Date: 2016-11-10
Speaker: Dr. L. E. Pirogov
Venue: Room - 113, Thaltej Campus

Abstract

The W40 dense core is associated with one of the nearest H~II regions and contains a clumpy dust ring probably formed by the ''collect and collapse'' process. Our observations revealed differences in physical characteristics of the clumps and the effects of chemical differentiation. The western clumps show chemical composition typical for evolved low-mass cores while the chemistry of the eastern clumps of the ring most probably is influenced by the closely located main HII region. The eastern clumps are more massive than the western ones, have higher degree of turbulence and indications of infall motions. The interaction between ionized gas and neutral material at the outer boundary of the eastern branch of the ring apparently triggers a new phase of star formation.

Rotation and Magnetism in Magnetohydrodynamics Simulations of Stellar Convection Zones

Date: 2016-09-30
Speaker: Dr. Bidya Binay Karak
Venue: Room - 113, Thaltej Campus

Abstract

It is believed that the magnetohydrodynamics processes are responsible for producing the solar magnetic field and the associated phenomena. Unfortunately, due to extreme conditions in the solar convection zone realistic direct numerical simulation of magnetohydrodynamics is not possible. However, significant progress can be made by performing simulations in local cartesian geometry and by producing turbulence using a forcing function. By choosing this forcing to be helical and by imposing a large-scale shear in the simulation box, we can easily excite large- and small-scale dynamos when the fluid is sufficiently random. I shall show that this kind of simple setup allows us to understand some basic questions of the solar magnetic field. Later, I shall present some results from rotating global-convection simulations in spherical geometry of the solar convection zone. I shall show that under certain parameter regimes, these solar-like global simulations are successful in reproducing a few basic features about the solar cycle. Features and origin of the differential rotation produced in these simulations will also be discussed.

Symbiotic novae from 1.2m eye

Date: 2016-09-22
Speaker: Dr. Vishal Joshi
Venue: Room - 113, Thaltej Campus

Abstract

Symbiotic novae is a rare subclass of novae in which secondary is a late type evolved star(instead of main sequence in case of classical novae). Unlike classical novae, such systems are not compact and material accrete onto white dwarf via secondary wind. Explosion of symbiotic novae are very interesting as ejecta interacts with circumbinary matter and shock forms. Such shock is thought to be to production site of X-rays and also Gamma-rays which are detected recently in very few novae. In this talk, I will mainly discuss recent results on symbiotic novae observed from MIRO 1.2m telescope.

Galaxy Formation and Evolution in the JWST/TMT Era

Date: 2016-09-15
Speaker: Dr. Nimish Hathi
Venue: Room - 114, Thaltej Campus

Abstract

A comprehensive analysis of star-forming galaxies (SFGs), including a crucial sub-population of Lyman alpha emitters (LAEs), at high redshifts (z~>2) using multi-wavelength photometry and deep spectroscopy is vital for understanding the physical processes that govern the star formation activity and galaxy assembly through cosmic time. Until now, such studies were limited to small number of galaxies because of the lack of large area, deep observations at high redshifts. With extensive multi-wavelength photometry from HST deep fields, and deep ground-based spectroscopy from ESO/VLT, we can now investigate physical properties of a large sample of SFGs/LAEs at z~>2. I will present results from recent photometric and spectroscopic studies of SFGs/LAEs at high redshift. These studies could lead to a more thorough investigation, over a much larger redshift range, using upcoming large telescopes such as JWST and TMT.

Results from the Mars Science Laboratory rover

Date: 2016-09-08
Speaker: Dr. Jeremie Lasue
Venue: Room - 114, Thaltej Campus

Abstract

Since its landing in August 2012, the Mars Science Laboratory (Curiosity) rover has just completed two full martian years (approximately 4 terrestrial years) of exploring Gale Crater on the surface of Mars. As of October 2014, the rover started its goal to study layered sediments in the central mound of the crater. Before that, the mission drove and studied several areas on its way to the central Mound. Along the way, the rover observed evidence for past liquid water at the surface in the form of aqueously-altered minerals and sedimentary rock types indicative of transport in flowing water. In making these discoveries, the mission has already accomplished its overarching goal of finding evidence for past habitable environments on Mars. This talk will present highlights from the exploration, with particular emphasis on a tactical instrument on the rover called the ChemCam (Chemistry Camera) instrument. The result of collaboration between France and the United States, ChemCam includes an integrated remote Laser-Induced Breakdown Spectrometer (LIBS) and Remote Micro-Imager (RMI). The LIBS technique determines the elemental composition of a sample from the spectra emitted by a plasma of materials ablated by a high energy laser focused beam. The RMI is integrated into the telescope from which high resolution geological context images are recorded.

MFOSC-P - A Spectrometer-Camera for Mt. Abu 1.2m Telescope : The Optical Design

Date: 2016-08-18
Speaker: Dr. Mudit Srivastava
Venue: Room - 114, Thaltej Campus

Abstract

Mt. Abu Faint Object Spectrometer and Camera-Pathfinder (MFOSC-P) is envisioned as a general purpose user’s instrument providing both the imaging as well spectroscopy facilities within the same optical chain. The instrument is being designed as a pathfinder instrument for the FOSC on upcoming 2.5m PRL Telescope. Optics of the instrument has been designed for the visible wavelengths to provide spatial sampling of 3 pixels per arc-second of the seeing limited images. In spectroscopy mode two base resolutions of ~2000 and ~1000 using gratings/grisms would be provided. In the talk the speaker would discuss the details of optical design of the instrument as well as current status and future plans.

Photometric Studies on open star clusters

Date: 2016-08-11
Speaker: Dr. Devendra Bisht
Venue: Room - 114, Thaltej Campus

Abstract

Open clusters (OCLs) are excellent objects to investigate spiral arm structure, chemical composition, stellar population, dynamical evolution and star formation in the Galaxy. As we know, more than half of the currently catalogued open clusters (~3000) have been poorly studied or even not studied at all. Therefore, it is very essential to study the unexplored open star clusters to know the complete picture of the Galactic disk. This study includes the estimation of their physical parameters like radius, age, distance, reddening and metallicity. Here we present the broad band UBVI CCD photometry for several clusters using 104 cm Sampurnanand Telescope located at ARIES, Manora Peak, Nainital, India. The optical CCD data for these clusters are obtained for the first time. For open clusters Teutsch 126, Teutsch, 54, Teutsch 61 and Czernik 3 we hae used 2MASS JHKs data. The stellar density distributions and color-magnitude diagrams are used to determine their structural parameters (cluster center, cluster radius, core radius, tidal radius, Galactocenteric coordinates and the distance from the Galactic plane). We have estimated the age, distance, reddening, colour excess, total mass and dynamical relaxation time of these open star clusters as well.

Can the jets showing bends or wiggles be relativistically beamed?

Date: 2016-08-04
Speaker: Prof. Ashok Singal
Venue: Room - 114, Thaltej Campus

Abstract

AGNs show relativistically moving jets, which many times show bends and wiggles on many scales. The conventional wisdom is that these jets appear very bright because they are oriented close to the line of sight of the observer and that relativistic beaming makes them look much brighter than they really are in their respective rest-frames. Now any misalignments seen in these jets would imply a change in the orientation angle, which should cause a large change in the beaming factor. But the bends and wiggles do not show such high contrasts. Is the appearances of such bends be really consistent with that the jets are relativistic? Are we missing something? In this talk I explore jets at different orientation angles with respect to the observer and thereby with different Doppler factors and investigate their effects on the intensities and appearances to the observer. It may be kept in mind that sometimes a small change in angle in the intrinsic frame might appear much larger due to the projection, more so when seen close to the line of sight. Therefore I take into account various projection effects in the jet appearance.

Journey of Binary Black Holes: From Supercomputer to LIGO to Universe

Date: 2016-07-29
Speaker: Mr. Karan Jani
Venue: Room - 114, Thaltej Campus

Abstract

The historic detections of binary black hole mergers in the first science run of Advanced LIGO has inaugurated the era of Gravitational Wave Astronomy. In this talk I recap the new astrophysics we have learned about black holes from these gravitational waves, and narrate our search plan to hunt for binary black holes as massive as 600 solar masses and up to cosmological distance of 10 Gpc. I emphasize the role of supercomputer simulations of solutions to Einstein Equations has played in confirming these gravitational wave signals as originating from black hole merger, and present a roadmap on how these simulations would guide future detections and hints of physics beyond General Relativity.

Unveiling the nature of Infrared-Faint Radio Sources (IFRSs)

Date: 2016-07-21
Speaker: Dr. Veeresh Singh
Venue: Room - 114, Thaltej Campus

Abstract

Recent deep radio surveys combined with auxiliary InfraRed (IR) surveys have discovered a new population called ‘Infrared-Faint Radio Sources’ (IFRSs) that are relatively bright radio sources with faint or no counterparts in IR and optical wavelengths. Given the dearth of more sensitive optical and IR data, the initial investigations on IFRSs were limited to radio wavelengths. We identify and investigate the nature of IFRSs in deep extragalactic fields, wherein deep optical (m_r ~ 25; Subaru, Very Large Telescope), near-IR (m_K ~ 23; UKIDSS, VIDEO), mid-IR ([3.6] ~ 2 microJy, Spitzer) data are available, along with the deep radio data. Our results suggest that IFRSs population constitute primarily radio-loud Active Galactic Nuclei (AGN) at higher redshifts (z ~ 1.7 - 4.0). In general, IFRSs exhibit compact radio emission with steeper radio spectra and thus suggesting them to be young radio galaxies in their early phase of evolution. Given the faintness or non-detection of IFRSs in optical/IR/X-ray wavelengths, it is plausible that IFRSs represent the population of obscured AGN as postulated by the modelling of X-ray extragalactic background emission.

Cooling and heating mechanism in cool-core cluster galaxies: A case study of ZwCl 2701

Date: 2016-07-15
Speaker: Dr. Nilkanth Vagshette
Venue: Room - 114, Thaltej Campus

Abstract

Observational evidences of cooling and heating mechanism are now available in majority of galaxy clusters. Heating from Active Galactic Nuclei (AGNs) is considered to be the prime candidate to offset/quench the cooling flow. We performed a systematic search for such heating mechanism (cavities/bubbles) in hot gas of galaxy clusters (specifically in ZwCl 2701) by using Chandra archival data. These observations of ZwCl 2701 show the presence of an extensive pair of ellipsoidal cavities along the East and West directions within the central region of <20 kpc. Detection of bright rims around the cavities suggested that the radio lobes displaced X-ray emitting hot gas forming shell-like structures. Comparable values of cavity power and cooling luminosity of ZwCl 2701 suggested that the mechanical power of the AGN outburst is large enough to balance the radiative cooling in the system. The star formation rate derived from the H_alpha luminosity was found to be 0.60 M_sun/yr, which is about three orders of magnitude lower than the cooling rate of ~ 196 M_sun/yr. The details will be discussed in the seminar.

Properties of high mass X-ray binary pulsars

Date: 2016-06-23
Speaker: Gaurava K. Jaisawal
Venue: Room - 114, Thaltej Campus

Abstract

The accretion powered X-ray pulsars are rotating neutron stars that powered in X-rays as a result of mass accretion from the optical companion. They are consider to be ideal astrophysical laboratory to test the fundamental properties of matter under extreme conditions. During the process of accretion, the interaction between pulsar's magnetic field and accreting gas results in the formation of a column like structure on the poles of neutron star that acts as the source of X-ray emissions. The accretion column has complicated geometry, hosting numerous complex processes that shape the broad-band continuum of these objects. In this talk, I will discuss about these mechanisms, emission beam patterns, properties of pulse profiles and observational evidences to directly estimate and probe the magnetic field of these pulsars.

Decade long optical monitoring of blazar 3C66A

Date: 2016-06-16
Speaker: Mr.Sameer
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Blazars are radio loud AGNs with their relativistic jets pointed close to line of sight of the observer boosting their emission due to relativistic beaming. They show rapid and strong variability spanning the entire electromagnetic spectrum, high and variable polarization in radio and optical regimes and dominant non-thermal emission from radio to high energy gamma-rays. These also show long term variations with varying time scales. It would be interesting to see if the short term variations have any relationship with the long term trends. Keeping this in mind, we have been monitoring a set of blazars from Mt Abu IR Observatory (MIRO) for more than 10 years now. In this presentation, I would like to discuss this program, with specific study of BL Lac Object 3C66A. The monitoring of blazar 3C66A, from 2005 November to 2016 February, in the B, V, R, I broad bands using 1.2m telescope of MIRO was carried. Blazar 3C66A exhibited significant variations in optical flux on short and long term time scales. However, it showed a IDV duty cycle of about 16% only. Our statistical study, using structure function, discrete correlation functions and L-S periodogram suggests time scales of intra- night variability from ~ 30 min to about 3.5 hours, and, in at least one case existence of quasi-perodic oscillations with a period of ~ 2.5 hr. The IDV amplitudes in R-band were found to vary from 0.02 mag to as high as 1.1 mag. The typical rate of flux variation was estimated to be ~0.06 mag/hr in both the rising and falling phases. The shortest timescale of variation results in setting an upper limit of 8.1 x 10^14 cm for emitting region size and about 4.3 x 10^8 Msun as mass of black hole.

Planet Formation in Dense Star Clusters

Date: 2016-06-02
Speaker: Dr. Henry Throop
Venue: Seminar Room - 113 (Thaltej Campus)

Abstract

Our Solar System's closest stellar neighbors are several light years away, and most models of the Solar System's formation assume that we have been isolated from other stars. But a growing body of evidence suggests that most stars and planets form in star clusters far more dense, where tens of thousands of young stars are initially packed into the same volume of space as between us and our closest stellar neighbors. The Orion region is one such nearby dense stellar cluster, where UV radiation, clouds of gas, and encounters with other stars can shape the future of planetary systems. I will discuss our recent observations and modeling of the formation of stars, disks, and planets in dense star clusters such as Orion, and what this says about the formation of stars and planets throughout the galaxy.

Soft X-Ray Telescope onboard ASTROSAT and Study of Blazars

Date: 2016-05-23
Speaker: Dr. Sunil Chandra
Venue: Seminar Room-113 (Thaltej Campus)

Abstract

Soft X-ray telescope (SXT) is a focusing X-ray telescope onboard the first Indian space-based multi-wavelength observatory ASTROSAT. The focusing optics of SXT, approximate Wolter-I type configuration, is capable to enable us X-ray imaging in 0.3-8.0 keV band. The focal plane camera is very similar to the one flown with Swift XRT. The SXT along with other instruments onboard ASTROSAT namely, UVIT, LAXPC and CZTI, becomes a very unique astronomical facility which can simultaneously cover a very broad energy range (130-530 nm & 0.3-150 keV) of spectrum. Since its launch on 28 September 2015, the instruments onboard ASTROSAT are producing very exciting results for a variety of astronomical sources for example AGNs, XRBs, Clusters etc. Blazars, because of their emission ranging over almost complete spectrum, are one of the key sources for ASTROSAT science. The simultaneous observations of emission produced because of two different processes (Synchrotron and SSC ) will enable us to improve the basic understanding of processes in these objects. In this talk I will present few preliminary results obtained from the SXT observations.

Magnetic Field in High Mass X-ray Binaries

Date: 2016-04-28
Speaker: Mr. Prahlad Epili
Venue: Seminar Room -113 (Thaltej Campus)

Abstract

X-ray Binaries (XRBs) are the astrophysical systems powered by accretion onto a compact object which gravitationally captures part of the mass lost by its companion star. Many of the observed properties of XRBs depend upon the mass of the companion star, which classifies the XRBs as high mass X-ray binaries (HMXBs) and low mass X-ray binaries (LMXBs). Among the HMXBs, the largest population are Be/X-ray binaries consisting of NSs with Be type companions. The accretion in these systems is mainly quasi-spherical wind/ disk accretion. During the episodes of giant outbursts, the pulse period in these systems show significant spin-up. Recently in October 2013, KS 1947+300 (a Be/X-ray binary) gone through a giant outburst with peak luminosity reaching ~1038 erg s-1 (with a mass accretion rate ~ 1018 g/s). It showed a significant spin-up in the NS pulse period. We have used its spin-up rate to estimate the magnetic field of the pulsar by using the theory of settling quasi-spherical accretion. The details of pulse-period variations and evolution of X-ray pulse profiles of KS 1947+300 during the giant outburst with subsequent changes in the spectral parameters over the pulse phase will be discussed.

Magnetic Field in High Mass X-ray Binaries

Date: 2016-04-28
Speaker: Prahlad Epili
Venue: Room - 114, Thaltej Campus

Abstract

X-ray Binaries (XRBs) are the astrophysical systems powered by accretion onto a compact object which gravitationally captures part of the mass lost by its companion star. Many of the observed properties of XRBs depend upon the mass of the companion star, which classifies the XRBs as high mass X-ray binaries (HMXBs) and low mass X-ray binaries (LMXBs). Among the HMXBs, the largest population are Be/X-ray binaries consisting of NSs with Be type companions. The accretion in these systems is mainly quasi-spherical wind/ disk accretion. During the episodes of giant outbursts, the pulse period in these systems show significant spin-up. Recently in October 2013, KS 1947+300 (a Be/X-ray binary) gone through a giant outburst with peak luminosity reaching ~1038 erg s-1 (with a mass accretion rate ~ 1018 g/s). It showed a significant spin-up in the NS pulse period. We have used its spin-up rate to estimate the magnetic field of the pulsar by using the theory of settling quasi-spherical accretion. The details of pulse-period variations and evolution of X-ray pulse profiles of KS 1947+300 during the giant outburst with subsequent changes in the spectral parameters over the pulse phase will be discussed.

Role of waves and small-scale transients in the heating of solar atmosphere

Date: 2016-04-07
Speaker: Dr. Girjesh R. Gupta
Venue: Room-114, Thaltej Campus

Abstract

The tenuous outer atmosphere of the Sun commonly known as 'corona', is orders of magnitude hotter than the solar surface. Heating of the solar corona along with acceleration of solar wind remain two of the most puzzling problems in the solar and space physics. There are several theories proposed to explain the phenomena, however, to identify any one dominant process is extremely difficult to do. Magnetohydrodynamics (MHD) waves and magnetic reconnection resulting in to transient release of energy, are currently the most promising heating models. In this presentation, speaker will mainly discuss different wave modes possible in the solar corona and their observational signatures. Some recent observational examples of propagation, and damping of MHD waves in the solar atmosphere will be presented. Contribution of energy carried by these waves in the context of heating of solar corona and acceleration of solar wind will also be discussed. Few examples of small-scale transient energy release events due to magnetic reconnection and their importance in the heating of solar atmosphere will also be presented.

Strong Hemisperic Asymmetry can Trigger Parity Changes in the Sunspot Cycle

Date: 2016-03-31
Speaker: Dr. Soumitra Hazra
Venue: Room-114, Thaltej Campus

Abstract

Although sunspots have been systematically observed on the Sun's surface over the last four centuries, their magnetic properties have been revealed and documented only since the early 1900s. Sunspots typically appear in pairs of opposite magnetic polarity which have a systematic orientation. This polarity orientation is opposite across the equator -- a trend that has persisted over the last century since observations of sunspot magnetic fields exist. Taken together with the configuration of the global poloidal field of the Sun -- that governs the heliospheric open flux and cosmic ray flux at Earth -- this phenomena is consistent with the dipolar parity state of an underlying magnetohydrodynamic dynamo mechanism. Although, hemispheric asymmetry in the emergence of sunspots is observed in the Sun, a parity shift has never been observed. We simulate hemispheric asymmetry through introduction of random fluctuations in a computational dynamo model of the solar cycle and demonstrate that changes in parity are indeed possible over long time-scales. In particular, we find that a parity shift in the underlying nature of the sunspot cycle is more likely to occur when sunspot activity dominates in any one hemisphere for a time which is significantly longer compared to the sunspot cycle period. Our simulations suggest that the sunspot cycle may have resided in quadrupolar parity states in the distant past, and provides a possible pathway for predicting parity flips in the future.

A multi-wavelength study of star formation activity in the S235 complex

Date: 2016-03-17
Speaker: Dr. Lokesh Dewangan
Venue: Room-114, Thaltej Campus

Abstract

The energetics of massive stars (M > 8 Msun) can strongly influence the surroundings. Massive stars can destroy star-forming clouds (i.e., negative feedback) and can also trigger star formation (i.e., positive feedback), leading to the formation of a new generation of stars including young massive star(s). In this talk, I will present results of a multi-wavelength analysis of a nearby massive star-forming S235 complex, helping to understand the feedback of a massive (O-type) star. The S235 complex has a sphere-like shell appearance at wavelengths longer than 2 micron and harbors an O9.5V type star. The position-velocity analysis of CO reveals an almost semi-ring like structure, suggesting an expanding HII region. The CO and dust continuum emissions trace eight embedded subregions, and five of them appear to be distributed in an almost regularly spaced manner along the sphere-like shell surrounding the ionized emission. We find that the dust clump masses increase as we move away from the location of the ionizing star. Photometric analysis reveals the clusters of young sources that are very well correlated with the dust clumps and CO gas. Together, all these results are interpreted as observational evidence of positive feedback of a massive star.

Planet Hunting & The Heritage of PARAS

Date: 2016-03-11
Speaker: Ms. Arpita Roy
Venue: Room-114, Thaltej Campus

Abstract

Twenty years ago, astronomers discovered the first exoplanet around a Sun-like star. Since then, we have become prolific exoplanet hunters and the field of radial velocity exoplanet detection has made tremendous strides towards ever-increasing precision. Pushing into extreme levels of precision (~10cm/s), we are now on the brink of discovering true Earth analogs, which will redefine our understanding of planetary habitability, and present the first realistic targets for discovering non-terrestrial life in the Universe. Highly stabilised, high-resolution, efficient spectrographs with adequate telescope allotments are required for this unique high-reward science goal. I will review the current state of the field for RV studies, and the value of instruments like PRL's PARAS spectrograph in the global effort to find and characterise exoplanets. Further, PARAS provides critical heritage and lessons for the next generation of precision instruments like the Habitable Zone Planet Finder (HPF, currently being assembled) and the NASA Extreme Precision Doppler Spectrograph (EPDS, in competition for award), both of which I am working on at Penn State. The current performance of PARAS promises great utility in the near future, as a workhorse spectrograph for they follow-up of TESS and PLATO candidates. In concert with HPF and EPDS, PARAS will play a powerful role in defining targets for future direct detection missions like JWST and HabEx.

Understanding the Luminous Supernovae and the role of small-size telescopes

Date: 2016-03-10
Speaker: Dr. Rupak Roy
Venue: Room-114, Thaltej Campus

Abstract

This is now established that the life of every massive star ends through a violent explosion, know as Core-Collapse Supernova (CCSN). In last couple of decades both theories and observational skills had been developed to understand the diversities of the CCSNe along with their origins and after-effects. In the most common scenario the core of such a star collapses under self-gravity and forms a compact object, while the outer shells are thrown away in the form of a cosmic catastrophic event. After the year 2005, dedicated surveys like CRTS, LOSS, ROTSE, PTF, PanSTARRS have discovered several events which have an average absolute Visual-band peak magnitude of about -21 mag, more than 2 mag brighter than CCSNe. These are Superluminous SNe (SLSNe). It seems that SLSNe are neither occurring in the nearby universe, nor are they hosted in massive-galaxies like our Milky Way. The mechanism for SLSNe is still not well understood. The circumstellar interaction or emergence of a magnetar after core-collapse or disruption of a massive star through pair production are the proposed scenarios. It is also not clear whether by origin the progenitors of CCSNe and SLSNe are same or are they different ? Recent research revealed that there are few events which come in between these two classes. These gap-transients are important to constrain the nature of the progenitors of these two different populations as well as their environments and explosion mechanisms. Here, we will discuss about the impact of circumstellar interaction on the observed properties of luminous supernovae and the importance of such study to understand the massive stars in the near and far universe. We will also discuss about the probability to detect such objects at early phases from the ground and space-based observatories and the role of small-size telescopes in this regard.

Looking at Ultraluminous X-ray sources and Cataclysmic Variables through the hard X-ray window using NuSTAR

Date: 2016-02-18
Speaker: Dr. Vikram Rana
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

The Nuclear Spectroscopic Telescope Array (NuSTAR) is the first mission to carry focusing high-energy X-ray telescope in space. NuSTAR, with its unprecedented combination of sensitivity, spatial and spectral resolution, turned out to be an ideal observatory to probe the hard X-ray properties of Ultraluminous X-ray sources (ULXs) and Cataclysmic Variables (CVs) as well as other X-ray binaries. ULXs are of prime interest, as they may be associated with a poorly explained regime of super-Eddington accretion onto a stellar mass black hole (BH), or even contain sub-Eddington intermediate mass BHs. Broad-band X-ray study of a sample of bright ULXs with XMM-Newton and NuSTAR revealed that all the sources in the sample show a clear cutoff at or above ~10 keV, strongly suggesting different physical origin for their X-ray emission compared to Galactic black hole X-ray binaries and supporting exotic super-Eddington modes of accretion. CVs, on the other hand, contain an accreting white dwarf as a compact object. NuSTAR provided a first direct detection of reflection component in three magnetic CVs, namely, V709 Cas, Ny Lup and V1223 Sgr. In the past, there was only an indirect evidence for the existence of reflection component based on presence of Fe fluorescence line. NuSTAR observations provided an important piece of information required to obtain a complete picture of these magnetic CVs. During this talk, I will discuss recent interesting science results on ULXs and CVs using hard X-ray data from NuSTAR and simultaneous soft X-ray coverage with XMM-Newton and/or Suzaku.

Evolution of models proposed for jet emission in blazars

Date: 2016-02-11
Speaker: Ms. Navpreet Kaur
Venue: Seminar Room # 113/114 (Thaltej Campus)

Abstract

Blazars, a sub-class of AGN (Active Galactic Nuclei), are known to show featureless spectra due to jet dominated emission owing to the peculiar orientation of jet axis towards us. They prove to be the most violent and highly energetic sources in the Universe because of greatly amplified intrinsic effect by relativistic boosting. Understanding their energetics and emission processes has been a challenge for astrophysicists and attempts were made to constrain the jet physics based on observational light curves. With advancement in observational technology and the establishment of multi-wavelength campaigns, it became difficult to explain the observations by those earlier existing models. In the talk, I would like to briefly summarize the evolution of blazar jet models proposed to explain physical processes and structure along with some of our observations.

Moving with the Comet - Science Results Update from Rosetta

Date: 2016-02-04
Speaker: Mr. Kumar Venkataramani
Venue: Room # 006 (Thaltej Campus)

Abstract

It has been a long journey for the International Rosetta Mission, from getting the approval in November 1993 to May 2014, when the spacecraft first reached the comet 67P/ Churyumov?Gerasimenko and to November 2014, when philae made a soft landing on the comet, the first ever of its kind. All through, it has been sending back wonderful data and high resolution images. Apart from missions like Stardust, Deep Impact and many other flyby's, for the first time ever, the comet's nucleus was imaged with high resolution cameras like OSIRIS on board the space-craft and the nuclear surface was probed directly by the lander. More recently, VIRTIS spectrometer on board the spacecraft has confirmed detections of water ice on the nucleus of the Comet. There are many more such interesting results coming from the so called Europe's comet chaser. These results can be a guiding step even for our ground based observations of various other comets. I would like to briefly discuss and summarize some of them in the talk.

Astrosat CZTI: Pre-flight calibration to in-flight operation

Date: 2016-01-28
Speaker: Mr. Mithun Neelkandan
Venue: Room # 006 (Thaltej Campus)

Abstract

Astrosat, India's first observatory class mission for multi-wavelength Astronomy was launched on 28th September 2015. Cadmium Zinc Telluride Imager (CZTI) is one among the five instruments on-board Astrosat. The instrument is composed of a large array of pixelated CZT detectors. Equipped with coded aperture mask it is capable of imaging and spectroscopy in 20-150 keV energy range. In this talk, I will present the details of ground calibration, response modeling and in-flight performance of the instrument. Some preliminary results from CZTI will also be discussed.

Very Low Mass stars and Exoplanets

Date: 2016-01-21
Speaker: Dr. Arvind Singh Rajpurohit
Venue: Room # 006 (Thaltej Campus)

Abstract

Very Low-Mass stars (M dwarfs) are an important source of information for probing the low mass end of the main sequence, down to the hydrogen burning limit. They are the dominant stellar component of the Galaxy. Moreover, an increasing number of M dwarfs are now known to host exoplanets, including super-Earth exoplanets. The determination of the accurate fundamental parameters for M dwarfs has therefore relevant implications for both stellar and Galactic astronomy as well as planetology. In this talk I will focus on spectroscopic approach using updated model atmosphere with the aim of obtaining their precise parameters. Further, I will discuss about the recent advances in both observations and theory of Hot-Jupiter atmospheres.

Mid-infrared dust and molecular features in Post-Asymptotic Giant Branch/Proto-Planetary Nebulae

Date: 2016-01-07
Speaker: Mr. V. Venkataraman
Venue: Room # 006 (Thaltej Campus)

Abstract

Stars with low and intermediate mass evolve along the Asymptotic Giant Branch (AGB) after their core hydrogen and helium burning. As the star evolves along the AGB phase it loses mass at an increasing rate until it is totally obscured by its own circumstellar envelope. As the central star evolves to higher temperatures and the circumstellar envelope gets detached from the stellar photosphere and drifts far off, its total spectral energy distribution (SED) becomes 'double-peaked' suggesting the onset of post-AGB (PAGB) or proto-planetary nebulae (PPNe) phase of evolution. The mid-infrared (IR) spectra and modeling of SED on a large sample of PAGB/PPNe objects will be presented in the talk. The mid-IR spectra of these samples showed a variety of features including the stretching and bending vibrations of polycyclic aromatic hydrocarbons (PAHs). The correlation of the derived physical and circumstellar parameters from the SED models with the strength of the observed PAH features will be discussed. The first rare detection of fullerenes in one of the source and its implication on the circumstellar chemistry will be discussed.

2015

Heating and Dynamics of the Solar Active Regions

Date: 2015-12-31
Speaker: Prof. Durgesh Tripathi
Venue: Room # 006 (Thaltej Campus)

Abstract

A radio relic in CIZA J0649.3+1801 ?

Date: 2015-12-17
Speaker: Ms. Kamlesh Rajpurohit
Venue: Room # 006 (Thaltej Campus)

Abstract

Galaxy clusters form through a sequence of mergers of smaller galaxy clusters and groups. During mergers large scale shocks are driven into the intracluster medium (ICM), these shocks accelerate electrons to relativistic speeds. Together with magnetic fields these electrons emit synchrotron radiation and may form so-called radio relics. In observations radio relics are found to be elongated, peripheral, polarized, Mpc-scale diffuse synchrotron sources. Relics are generally found in merging systems supporting the scenario that they trace shock fronts. There is a evident that they have magnetic fields at the micro-gauss level but difficult to conclude. Their structure and origin is still enigmatic, however polarization plays a key role to distinguish them from other extragalactic objects. A diffuse elongated radio source has been found in a very less-studied galaxy cluster CIZA J0649.3+1801. I will present Westerbork Synthesis Radio Telescope (WSRT) observations of CIZA J0649.3+1801 at 18, 21 and 25 cm. We find that the source has a very steep spectrum (alpha=1.83), for radio relics it is unusual to have such a steep spectrum. On the other hand, the polarization analysis reveals that the source is polarized (8-14%) and has ordered magnetic field with B-vectors perpendicular to the source extension, favouring relic origin. Rotational measure (RM) maps shows strong galactic foreground emission in the direction of CIZA J0649.3+1801. We aim for unveiling the nature of the source by studying its Faraday spectra..

The Phenomenal 2015 Outburst of the Black-Hole X-ray Binary, V404 Cyg

Date: 2015-12-08
Speaker: Dr. Kunal Mooley
Venue: Room # 006 (Thaltej Campus)

Abstract

On 15 June 2015, the Swift Burst Alert Telescope detected an X-ray flare from V404 Cyg. The AMI-LA radio telescope was robotically triggered, revealing a bright and declining radio flare. This was the beginning of an energetic outburst that was to last for several weeks, at the end of which, the entire accretion disk of the X-ray binary would be blown away. An intense panchromatic observing campaign of the outburst was carried out using ground-based and space-based observatories, involving both professional and amateur astronomers worldwide. In this talk, I will present the observational data gathered during V404 Cyg's 2015 outburst, some of the early results from their analyses, and describe the X-ray and radio observations that have helped paint a stunning picture of the accretion and jet processes taking place during the outburst. I will conclude by giving the avenues where ASTROSAT and contemporary radio telescopes will be able to make significant advances in our understanding of X-ray binaries similar to V404 Cyg.

Phenomenology of active galaxies from parsec to megaparsec scale: A multiwavelength view

Date: 2015-12-03
Speaker: Dr. Veeresh Singh
Venue: Room # 006 (Thaltej Campus)

Abstract

Nearly all galaxies host SuperMassive Black Holes (SMBHs) of the order of million to few billion solar mass, in their centres. A galaxy is classified as active galaxy or Active Galactic Nuclei (AGN), when SMBH is in accreting phase, which in fact, depends on the availability of matter in its vicinity. SMBH accretes matter via formation of accretion disk and also ejects a fraction of accreted matter via outflowing bipolar collimated jets. In a subclass of AGN called radio galaxies, jets are powerful enough to plough through interstellar medium and span up to even megaparsec scale entrenching into intergalactic medium. We use multiwavelength observations of AGN to probe inner structures at parsec scales to outer structures up to megaparsec scales. X-ray analysis of AGN emission infers the presence of obscuring torus at parsec scales around AGN and thus validates orientation based unification model. While radio observations help us in probing jet-lobe structures from parsec to megaparsec scales. It is found that radio galaxies exhibit variety of radio morphologies that can be understood by evolutionary scenario in addition to viewing angle effects. Few radio galaxies are found to show two pairs of jet-lobes and thus offer a direct evidence for episodic AGN activity.

Innermost structure of quasars using optical interferometry and reverberation mapping

Date: 2015-11-04
Speaker: Dr. Suvendu Rakshit
Venue: Room # 006 (Thaltej Campus)

Abstract

The broad line region (BLR) of Active Galactic Nuclei (AGNs) contains the high velocity gas clouds transporting material from the dust torus to the accretion disk around the central super massive black hole (SMBH). Unveiling BLR structure is critical to understand the accretion mechanism driving the SMBH evolution and shaping the AGN inflows, outflows and jets. Reverberation Mapping (RM) constrains the BLR geometry, kinematics, mass and equivalent linear size with parameter degeneracies and fudge factors depending from the source geometry. Optical Interferometry (OI) yields independent constrains on BLR structure, mass and equivalent angular size. In this context I will present a 3D geometrical model and kinematical model of BLRs that simultaneous predicts all RM and OI signals, and the result of our first optical interferometric observation of BLR of 3C273. I will also talk about how to use quasars to estimate distances using "quasar parallax".

Radiation reaction and the pitch angle changes in a synchrotron source

Date: 2015-09-10
Speaker: Dr. Ashok K. Singal
Venue: Room # 006 (Thaltej Campus)

Abstract

In synchrotron radiation it is always assumed that the pitch angle of a charge remains constant during the radiation process. Using Larmor's radiation formula, Kardashev (1962) first presented the argument that as the radiation is beamed along the velocity vector of the charge, the momentum loss will also be along that direction and therefore the pitch angle of the charge would remain constant during the radiation process. The accordingly derived formulas are used for calculating energy losses of synchrotron electrons in radio galaxies and are the standard text-book material ever since. However, it turns out that this picture is not consistent with the special relativity and that in the case of synchrotron losses, the pitch angle in general varies. While the velocity component parallel to the magnetic field remains unaffected, the perpendicular component does reduce in magnitude due to radiative losses, implying a change in the pitch angle. This apparent paradox gets resolved and one gets a consistent picture only when effects on the charge motion are calculated from the Lorentz's radiation reaction formula. We derive the exact formula by taking into account the change of the pitch angle due to radiative losses. From this we first time derive the characteristic decay time of synchrotron electrons over which they turn from highly relativistic into mildly relativistic ones.

Imaging Spectrometers in Astronomy

Date: 2015-09-03
Speaker: Mr. Vaibhav Dixit,
Venue: Room # 006 (Thaltej Campus)

Abstract

Imaging Spectrometers in particular FOSC (Faint Object Spectrograph and Camera) kind of designs have become very popular within the astronomical community and they have been used on all major telescope facilities across globe. Due to their ability to easily switch between imaging, spectroscopy and polarimetry modes, these instruments have found huge application in various astronomical studies. Design of such instrument poses several technical challenges in their optical and opto-mechanical configurations. An optimum choice of system parameters like Field of View, Resolution and Spectral Range while still meeting the science goal, is very important while conceiving such instrument. In this talk the speaker will present a general description of the FOSC type instrument and will describe the optical and opto-mechanical design considerations.

Spatial distribution of neutral hydrogen in the Milky-way

Date: 2015-08-21
Speaker: Prof. Avinash Deshpande,
Venue: Room # 006 (Thaltej Campus)

Abstract

The atomic hydrogen in our Galaxy has structure on a range of spatial scales. The talk will briefly describe the various probes to study the HI distribution on small and large scales, as well as discuss some of the puzzles these studies created, and their possible resolution.

Observational Studies of Hydrogen Deficient Stars for Investigating their Evolutionary Connections

Date: 2015-08-20
Speaker: Dr. Hema B. P.,
Venue: Room # 006 (Thaltej Campus)

Abstract

R Coronae Borealis (RCB) stars and the hydrogenÂdeficient (H-deficient) carbon stars are the H-deficient and carbon rich F- and G-type supergiants. There are only handful of these stars known in the Galaxy. The origins and evolution of these stars are not yet clear. For my thesis, the origins and evolution of these stars were investigated, 1. By deriving the Galactic RCB/HdC stars' CÂabundances and the 12C/13C ratios, from C2 Swan bands using their highÂresolution spectra, which are potential clues to their origin, and 2. By conducting a lowÂresolution spectroscopic survey for identifying the HÂdeficient stars in the red giant sample of the Galactic globular cluster Omega Centauri, to pinÂpoint their position on the HRÂdiagram. In my talk I will be presenting the observations, methodology, analyses and results of our above mentioned studies.

Physical environment of massive star-forming region W42: a multi-scale and multi-wavelength approach

Date: 2015-07-31
Speaker: Dr. Lokesh Kumar Dewangan,
Venue: Room # 469 (Theory Seminar Hall, PRL Main Campus)

Abstract

In massive star formation research, it is extremely difficult to identify a promising sample of the precursors/early critical phases of O type massive stars. In the literature, there are only a handful of massive young stellar objects (MYSOs) known to be associated with highly collimated jets and/or Herbig-Haro (HH) objects. I will present results of an analysis of massive star-forming region W42 using a multi-scale and multi-wavelength approach. These results throw light on the physical environment of W42 on smaller (about 5000 AU) and larger (about 10 pc) scales, helping to understand the formation and interaction of massive stars with their surrounding environment. A bipolar appearance of W42 complex (on scales of about 10 pc) is evident due to the ionizing feedback from the O5-O6 type star in a medium that is highly inhomogeneous. The VLT/NACO adaptive-optics K and Lp images resolved this ionizing source into multiple point-like sources below ~5000 AU scale. In this complex, we have discovered a parsec scale H2 outflow that is driven by an infrared counterpart of the 6.7-GHz methanol maser emission (i.e. W42-MME; stellar mass 19 solar mass and extinction 48 mag). Furthermore, we have investigated a collimated jet-like feature (in the inner 4500 AU) in W42-MME using the VLT NIR adaptive-optics images. The jet is located inside an envelope/cavity (extent ~10,640 AU) that is tapered at both ends and is oriented along the north-south direction. Such observed morphology of the outflow cavity around the massive star is scarcely known and is very crucial for understanding the jet-outflow formation process in massive star formation.

Variability study of Blazars: A case study of S5 0716+714

Date: 2015-07-16
Speaker: Ms. Navpreet Kaur
Venue: Room # 113 (Thaltej Campus)

Abstract

One of the most spectacular, powerful and mysterious sources in Universe are Active Galactic Nuclei (AGN). Most of them are hosted by elliptical galaxies at high redshifts and contain supermassive black hole (SMBH) at their center. Depending upon their orientation with respect to observer, AGNs have been classified into various sub-classes, out of which, one sub-class is Blazar. Blazars have been known to have their relativistic jets pointed almost towards the line of sight and dominates their emission. Since jets are supposed to be launched from the close vicinity of black hole, blazar study provides an insight to probe deep into the central engine. These sources show emission at all wavelengths (radio - optical - NIR - X-rays up to gamma rays) which is variable at various time-scales ranging from years to few tens of minutes. They also show variability in both, total flux and polarization. Blazar S5 0716+714 is one of our favorite sources, which shows short-term and long-term variability and has been monitored by our group for more than 10 years in optical. Observations have been carried out by PRL's MIRO (Mount Abu InfraRed Observatory) facility at Gurushikhar, Mount Abu. We use two moderate sized telescopes; 1.2 m (f/13) Cassegrain type telescope and 50 cm (f/6.8) CDK20 system with Liquid nitrogen cooled optical CCD camera and EMCCD. We found the source showing unprecedented brightness in late January, 2015. Few results showing intra-night and inter-night variability during last two years for this source will be discussed in the talk.

Magnetic fields in triggered and spontaneous star forming regions

Date: 2015-07-13
Speaker: Dr. Archana Soam
Venue: Room # 113 (Thaltej Campus)

Abstract

The study of magnetic fields in molecular clouds formed in various environments have enormous potential towards understanding the underlying physics behind the role played by the magnetic fields in the formation of molecular clouds and the subsequent star formation. The closer and isolated molecular clouds provide the fascinating laboratories where the importance of magnetic fields can be explored towards ongoing activities like evolution of the cloud and star formation. My main focus is to explore the magnetic fields morphologies and strength in various environments of molecular clouds to understand the open issue of importance of magnetic fields in interstellar medium. The regions where I have mapped the magnetic fields are broadly classified into the categories viz. triggered star forming and spontaneous star forming sites. In triggered star forming regions, I have chosen the multiple bright-rimmed clouds (BRCs) and cometary globules (CGs) associated to some HII regions surrounded by a high mass star in the center. In a magnetic field dominated scenario for isolated and spontaneous low-mass star formation, the cores are envisaged to gradually condense out of a magnetically subcritical background cloud, through ambipolar diffusion. To understand the evolution of magnetic fields in the cores of different ages (ages determined based onto their chemical evolution), I have studied the magnetic fields in a number of starless/prestellar cores. Magnetic fields morphologies in some of the cores harboring very low luminosity objects (VeLLOs) have also been made. The magnetic field maps are made with the help of optical, NIR and submm polarimetric techniques using observing facilities available in India and abroad.

Understanding the properties of Be/X-ray binary pulsars during outburst

Date: 2015-07-09
Speaker: Prahlad R. Epili
Venue: Room # 113 (Thaltej Campus)

Abstract

Be/X-ray Binaries (BeXBs) are known to be a subgroup of high mass X-ray binaries. In these binaries, the compact object is generally a neutron star (pulsar) and the optical companion is known to be a Be star. These pulsars are undetectable in X-ray band during most of the times. Sudden enhancement in X-ray emission from these sources occurs and lasts for tens of days to a few months during which the pulsar becomes easily detectable with space based X-ray observatories. The neutron star in the BeXBs has moderately eccentric orbits (e ≥ 0.3). Hence during periastron passage, a sudden mass transfer onto the neutron star leads to an enhancement in X-ray flux (i.e. X-ray outbursts). BeXBs can have giant (type II) and normal (type I) outbursts in addition to long quiescence periods. Giant outbursts in these systems results in a transient increase in X-ray flux of about 10^3 - 10^4 times that at quiescence. The resultant luminosity reaches the Eddington luminosity for a neutron star, making the system the brightest in the X-ray sky. We have attempted to investigate the X-ray timing and spectral parameters of such a pulsar (i.e. KS 1947+300) during its giant outburst in 2013. The results of this work will be discussed along with the characteristics of Be X-ray binaries.

Study of Comet C/2014 Q2 (Lovejoy)

Date: 2015-07-02
Speaker: Kumar Venkataramani, PRL
Venue: Room # 113 (Thaltej Campus)

Abstract

Comets are cold icy bodies in the solar system that were probably formed in the solar nebula and are considered to be the signature bodies to understand the formation of the solar system. As the comet nucleus makes its journey towards its parent star, the ices start sublimating giving rise to a mixture of gas and dust which forms the coma. The comet spectrum shows a continuum in the visible region at large heliocentric distances which is basically the solar radiation scattered by the cometary dust. In addition it exhibits strong molecular emission lines close to the perihelion. Comet C/2014 Q2(Lovejoy), an Oort cloud comet was discovered by Terry Lovejoy in August 2014. The spectra of this comet were obtained using the low resolution grating spectrograph (LISA) on the 0.5m telescope at the Mount Abu Infrared Observatory. LISA is a slit based spectrograph with a focal ratio of F/5 covering a wavelength range of 400-700 nm in the visible and 650 nm to 1 micron in the Near IR. The spectra showed very strong molecular emission lines close to its perihelion in late January 2015. Some of these emission lines, the extent of the dust continuum and their variation with heliocentric distance were studied. The results from this study will be presented in the talk.

Timing and Spectral Studies of Supergiant High Mass X-ray Binary Pulsars

Date: 2015-06-25
Speaker: Gaurava K. Jaisawal
Venue: Room # 113 (Thaltej Campus)

Abstract

X-ray binary pulsars with supergiant stars as companions generally show random and rapid variation in X-ray intensity. Unlike Be/X-ray binary pulsars, the supergiant X-ray binary pulsars rarely show X-ray outbursts of significantly enhanced luminosity. In case of several such binary pulsars, variation in X-ray intensity is observed at time scales of hundreds to thousands of seconds. Soft and hard X-ray light curves of these pulsars show the presence of several flare like episodes along with extended low intensity segments. We tried to investigate the properties of a few such pulsars such as OAO 1657-415, 4U 1700-37 during low and high flux levels by using observations from space based X-ray observatory Suzaku. Our results show that the occurrence of flares in the X-ray light curves can be explained by using clumpy stellar wind model. The results obtained from these works will be discussed in the talk.

The spectral signature of stars

Date: 2015-06-18
Speaker: Ms. Priyanka Chaturvedi
Venue: Room # 113 (Thaltej Campus)

Abstract

The light that is emitted from the stellar photosphere consists of continua as well as absorption lines, which serves as a unique signature to help us decode the remarkable nature of stars. The behavior of these stellar absorption lines depend upon the surface temperature, mass, evolutionary status and chemical composition of the star. The study of stellar photosphere is a useful tool to connect our observations with other relevant parameters of interest. In the Radial Velocity technique, we determine the mass and radius of the secondary star (or a planet) as a function of the mass and radius of the primary host star. Thus, accurate determination of host star properties is important in proper estimation of mass and age of the companion. We, as a team of PARAS (Physical Research Laboratory Advanced Radial-velocity Abu-sky Search), have developed an indigenous package, PARAS-SPEC, to determine the stellar properties of the primary host star. It is an IDL based programming tool and based on the synthetic spectral fitting method and equivalent width method. The basic principles and methodology used for designing this tool will be discussed in the talk. A few results when applied to known and unknown stars shall be discussed along.

New results from PARAS: Evidence of exo-planets around Giants

Date: 2015-06-11
Speaker: Dr. Abhijit Chakraborty
Venue: Room # 113 (Thaltej Campus)

Abstract

There are about 560 known exoplanets, which are as massive as Jupiter or bigger, and this is about one-third of the known population of exoplanets. Most of these exoplanets orbits around Sun-like dwarf stars. Despite large and diverse population of such Jupiters, only a handful of them are found around Giant stars (evolved stars, finished Hydrogen burning in their core) with an orbit of less than 0.5AU semi-major axis. Such evolved stars have typically radii of 2 to 10 solar radii and exoplanets in the close proximity usually gets destroyed due to very strong tidal interactions and heat and eventually gets engulfed. PARAS results show existence of a transiting Jupiter of mass 1.4 Jupiter mass and approximate period of 39 days around an evolved star of 1.5 Solar Mass (upper limit) with stellar radius of 2 to 4 solar radii. The planet has an orbital eccentricity of 0.05 and semi-major axis of 0.255 AU and measured K-value (semi-amplitude) of 65m/s. I will discuss about other prospective transiting candidates and also new results on tau Ceti a Radial-Velocity standard star showing unprecedented sub-1m/s precision as a future precursor of what can be achieved using the new PRL 2.5m telescope.

Determination of Hubble constant (H_0) through monitoring of gravitationally lensed quasars

Date: 2015-05-14
Speaker: Dr. S. Rathnakumar
Venue: Room # 113 (Thaltej Campus)

Abstract

The Hubble constant at the present epoch ($H_0$) is an important cosmological parameter on which all extragalactic distances depend. Multiple approaches need to be pursued to constrain its value in order to be able to identify and eliminate unknown systematic errors present in any one approach. Strong gravitational lensing offers one method to constrain $H_0$ free from calibrations associated with standard candles. It is based on (i) measuring the time delay(s) between the multiple images of a cosmologically distant variable source which is strongly lensed by a galaxy or galaxy cluster that lies in close proximity to its line-of-sight and (ii) modelling the mass distribution of the deflector. We have carried out photometric monitoring of a sample of six gravitationally lensed quasars using the 2-m Himalayan Chandra Telescope (HCT) as part of the COSmological MOnitoring of GRAvItational Lenses (COSMOGRAIL) campaign to measure time delays of most known lensed quasars in both hemispheres using a network of medium-size telescopes. I will discuss in detail our results of this campaign, using a newly devised "difference-smoothing" technique to measure time delay and pixellated modelling of the mass distribution of the deflector to constrain the $H_0$ value.

Lessons learnt from Herschel Gould Belt Survey Key project observations

Date: 2015-02-12
Speaker: Dr. Arabindo Roy
Venue: Room # 113 (Thaltej Campus)

Abstract

In this talk I will present two key results from the Herschel Gould Belt Survey (HGBS) -- 1) constrain on the submillimeter dust opacity and 2) the importance of filamentary structures in connection with prestellar core formation.Thermal dust emission is optically thin at sub-mm wavelengths and thus it is often used as a proxy of matter. The conversion of dust emission to dust mass depends on the adoption of the value of dust opacity. Although dust properties in the diffuse medium is relatively well understood, however, in the dense medium it is very poorly constrained. Theoretically, dust grains are expected to evolve in dense medium by accreting ice mantles and accumulating metals. Using dust continuum emission and near infrared extinction data we show an evidence for non-linear evolution of dust opacity in the moderately dense medium as a function of column density, suggestive of grain growth. For high column density (Av>20 mag), however, taking account of line of sight temperature variation is crucial for accurate deciphering of dust properties. We used Abel's integral transform on the B68 globule to disentangle temperature variation and compared dust opacity therein. This method also yielded sucessful results for the reconstruction of volume density and dust temperature structures. HGBS observations have demonstrated the importance of filamentary structures in connection with core formation. The interstelar MHD turbulence creates an ubiquitous network of filaments. Herschel has resolved these structures in the nearby molecular cloud and found that they maintain a constant inner-width of 0.1 pc irrespective of their stability state. Thermally supercritical filaments (M_line > 2c_s^2/G) harbor most of the prestellar cores and the derived core mass function (CMF) suggested that peak of the CMF corresponds to the Jeans mass of gravitationally unstable filaments, however, the origin of Salpeter like power-law slope towards high mass end was unclear. I will also discuss that the longitudinal perturbation modes seeded by Kolomogorov turbulence may explain the power-law part of the CMF.

High Energy Emission in Blazars

Date: 2015-01-27
Speaker: Mr. Pankaj Kushwaha
Venue: Room # 006 (Thaltej Campus)

Abstract

Blazars comprised of BL Lacs and flat spectrum radio quasar are radio-loud active galactic nuclei having a relativistic jet seen at very small angles to our line of sight. They are characterized by a rapid and highly variable non-thermal continuum emission across the electromagnetic spectrum dominated by jet emission with a distinctive double-hump spectral energy distribution profile. The low energy hump (IR-to-UV/X-rays) is the result of synchrotron emission from relativistic non-thermal electrons in the jet. However, the nature (leptons/hadrons) and origin (inverse Compton/proton-synchrotron/cascades) of high energy hump (X-rays-gamma-rays) is not very clear. The availability of almost simultaneous monitoring of a rich sample of blazars have made the blazar SED modelling one of the strongest tool to diagnose the physical processes in blazars. In this seminar, I will be presenting few of the interesting results obtained from my studies done during last couple of years to probe the high energy emission in blazars, particularly for OJ 287 and PKS 1222+216.

Optical Imaging / Instrumentation in Applied Sciences: Synergies and Ideas for Astronomical Instrumentation

Date: 2015-01-22
Speaker: Dr. Mudit Srivastava
Venue: Room # 113 (Thaltej Campus)

Abstract

Optical Imaging Techniques and instruments have been used extensively in several application domains, ranging from fundamental research (e.g. Astronomy and Astrophysics) to applied and commercial research e.g. Life sciences, bio-medical, semi conductors etc. These two extreme domains of applications offer different kind of constraints on the several important aspect of instrument development e.g. financial, technical, complexity, scope, life cycle, compliances etc. While instrmentation in pure science research is driven by objectives set by scientific queries, can be very complex, specialized and can only avail to a specified set of people/experimental groups, instrumentation for the other domain has to be user's friendly, and more important cost effective. In this talk, I would emphasize on the similarities in the instrumentation between these two domains and fews ideas that could be imported for the development of astronomical instrumentation.

Radio Continuum Studies of Wolf-Rayet Galaxies

Date: 2015-01-15
Speaker: Dr. Shweta Srivastava,
Venue: Room # 113 (Thaltej Campus)

Abstract

The study of Wolf - Rayet (WR) galaxies plays an important role in understanding the properties of the star-forming events. The WR galaxies typically exhibit spectral features of WR stars which are indicative of young star-burst. This talk will cover the observed spectral energy distribution from 150 MHz to 10 GHz in a sample of WR galaxies using data obtained from the Giant Metrewave Radio Telescope (GMRT) and literature. The distance to the galaxies range from 4 - 60 Mpc and optical size from 0.8 - 9 arc-min. When the mechanism such as emission, absorption and energy loss, becomes active in these galaxies, the radio spectrum found to exhibit a variety of spectral shapes.

2014

High Mass X-ray Binary Pulsars

Date: 2014-12-18
Speaker: Dr. Sachindra Naik
Venue: Room # 113 (Thaltej Campus)

Abstract

High Mass X-ray Binary (HMXB) pulsars are binary pulsars which consist of a neutron star (compact object) and a massive optical companion star orbiting around the common center of mass. Depending on the spectral-type of optical companion star, these pulsars are classified as Be/X-ray binary pulsars and Supergiant X-ray binary pulsars. Though mass accretion takes place from the massive optical companion star to the compact object, the difference in the mode of mass accretion is the cause of difference in the observed X-ray properties. Pulsars in these binary systems are highly variable in X-ray bands. Be/X-ray binary pulsars show regular and periodic X-ray outbursts whereas the pulsars in supergiant X-ray binaries show irregular and random variability in X-ray intensity. Timing and broad-band X-ray spectral properties of a few Be/X-ray binary pulsars and supergiant X-ray binary pulsars will be discussed. Along with X-ray properties, optical and infrared properties of the Be companion star (in Be/X-ray binary pulsars) will also be discussed.

Study of QPO in the black hole source GRS 1915+105 with NuSTAR data

Date: 2014-12-04
Speaker: Ms. Anjali Rao
Venue: Room # 113 (Thaltej Campus)

Abstract

Timing studies of the black hole binaries reveal the presence of the quasi periodic oscillations in the light curves. However, it is rather difficult to establish the oscillatory nature of the spectral component from the spectral studies. In this talk, we will discuss how the data from NuSTAR has helped to understand the link between quasi periodic oscillations and the spectral components. We studied a black hole source GRS 1915+105 with the NuSTAR data and the results will be discussed in the talk.

Energy conservation for a radiating charge in classical electrodynamics

Date: 2014-11-13
Speaker: Dr. Ashok K. Singal
Venue: Room # 113 (Thaltej Campus)

Abstract

It is shown that the well-known century-old disparity in classical electrodynamics between the power radiated in electromagnetic fields (from the familiar Larmor's formula, or in a relativistic case, from Li\'{e}nard's result) and the power-loss, as calculated from the radiation reaction (given by the Lorentz-Abraham-Dirac equation) on a charge undergoing a non-uniform motion, is successfully resolved when a proper distinction is made between the quantities expressed in terms of the retarded time and those expressed in terms of a ``real time''. It is shown that the expression for the real-time radiative power loss from a charged particle is somewhat different from the familiar formulas in the literature. A use of the standard formulation has resulted, among other things, in a potentially incorrect text-book formula (derived by Kardashev 1962 and in use ever since) for the life-times of electrons undergoing synchrotron losses in radio galaxies or supernova remnants. The correct formulation will be provided.

Declining solar magnetic fields and Low Frequency Radio Astronomy: Is there a link?

Date: 2014-10-30
Speaker: Prof. P. Janardhan
Venue: Room # 113 (Thaltej Campus)

Abstract

Observations of solar polar magnetic fields during the past three solar cycles viz. cycles 21, 22 and 23 have shown that the solar magnetic fields have been steadily declining since about 1995. This steady decline has also caused a similar decline in solar wind microturbulence levels. In recent times low frequency, meterwave radio astromomy is gaining importance. However, the ionospheric cut-off frequency of about 25 MHz sets a limit on the lowest frequncies one can observe from the ground. Ionosonde records of the cut-off frequency in solar cycle 23 indicates that the cut-off is also reducing and there is indication that it may go below 10 MHz. This talk will try to briefly look at these aspects.

Infrared Carbon Stars

Date: 2014-08-07
Speaker: Prof. N. M. Ashok, PRL
Venue: Room # 113 (Thaltej Campus)

Abstract

Infrared Carbon Stars(IRCSs) are carbon stars identified only by their infrared properties. The term Infrared Carbon Stars appeared in the literature as a consequence of one of the significant discoveries from the IRAS (Infrared Astronomy Satellite) mission. IRCSs are usually invisible or very faint in the optical. Their energy distribution is characterized by the emission from the dust grains contained in the circumstellar envelope that absorb the radiation from the central star which makes the detection at optical wavelengths difficult. The IRCSs are important objects to understand the evolutionary sequence in the asymptotic giant branch(AGB) and post-AGB phases of carbon rich stars. The photometric and spectroscopic properties in the infrared will be discussed.

PRL Optical Polarimeters

Date: 2014-07-31
Speaker: Dr. Shashikiran Ganesh, PRL
Venue: Room # 113 (Thaltej Campus)

Abstract

PRL's photo-polarimeter has been in use for nearly 3 decades and has seen a lot of changes in acquisition technology since the initial days. The last upgrades and developments will be discussed and some results presented. In addition to the photo-polarimeter we have recently developed two imaging polarimeters using EMCCD cameras as detectors. One of these is being used on the 0.5m CDK telescope while the other is being developed for use on the 1.2m telescope. Both these new instruments use different approaches to polarization measurement and I shall describe the respective setups and principles of operation. Some lab characterization carried out and results from test observing runs will be presented.

Hard X-ray polarimetry with Astrosat-CZTI – a new window in Astrophysics about to open?

Date: 2014-07-10
Speaker: Mr. Tanmoy Chattopadhyay, PRL
Venue: Room # 113 (Thaltej Campus)

Abstract

Astrosat is the forthcoming Indian satellite dedicated for astronomical studies. CdZnTe Imager (CZTI), one of the five instruments in Astrosat, is a pixelated spectroscopic instrument sensitive in 20-150 kev range. Because of its pixilation nature and significant Compton scattering efficiency at energies beyond 100 kev, it’s possible to measure polarization in X-rays exploiting the Compton scattering technique at energies beyond its primary energy range. In this talk, I will explain the simulation and experimental results which clearly show that the flight configuration of Astrosat-CZTI will be capable of polarisation measurement in hard X-rays and perhaps going to be the most sensitive polarimeter in the sky. This is the first ever experimental demonstration of the concept of doing Compton polarimetry by pixelated CZTI detectors. One of the main science goals of CZTI as a polarimeter is to differentiate various possible emission mechanisms in black hole sources in hard X-rays. In this talk, I will explain these processes and the possibility of constraining them by the fourth parameter of radiation.

M dwarfs in Eclipsing Binary Systems

Date: 2014-06-27
Speaker: Ms. Priyanka Chaturvedi, PRL
Venue: Room # 113 (Thaltej Campus)

Abstract

Despite copious number of M dwarfs present in our galaxy, masses and radii for these stars are still not determined at higher accuracies, primarily due to their fainter magnitudes in visible band. Radii for stars less massive than Sun are known to be 10% smaller theoretically than observed and temperatures are 5% higher. It is suggested that strong magnetic fields inhibit convection, which causes these stars to inflate. In general M dwarfs form an interesting area for stellar astronomers. We have observed few M dwarfs in eclipsing binary systems as companions to brighter stars. I will be discussing the techniques and some preliminary results obtained after studying these systems.

X-Ray/UV/Opt correlations in Seyfert Galaxies in the view of Disk-Coronae connections, a case study of NGC 4395

Date: 2014-06-19
Speaker: Dr. Sunil Chandra, PRL
Venue: Room # 113 (Thaltej Campus)

Abstract

The Seyferts or Seyfert Galaxies are a subclass of Active Galactic Nuclei (AGNs). These are well known to show emission dominant spectra unlike the typical stellar spectra. Based on the presence and absence of broad emission lines in their optical spectra, these galaxies are sub-classified in two branches 1) Type-I and 2.) Type-II. The strong emission lines show that our line of sight is far off to the relativistic jet. The major source of emission in Seyferts is the disk emission. The disk emission when interact with the violent gas revolving around the central engine give rise to strong emission lines. These regions are termed as line emission regions (namely BLRs & NLRs). On the other hand the disk emission when reprocessed by hot tenuous corona give rise the major X-ray emission. These object show the variability in flux over a range of frequencies. However, The origin of UV/Opt variability in AGNs is still under debate. Two well accepted physical mechanisms explaining these are (i) reprocessing of X-ray emission by the accretion disc and (ii) intrinsic variations of the thermal disk emission. Therefore, correlations in the X-Ray and UV/Opt variability may provide a better clue about the physics at work. In this talk I will be briefing about the general properties of Seyferts, geometry and current understanding of physical processes responsible for emission. The preliminary results of our collaborative work shows a supportive evidence of reprocessing model.

Large relative motion observed between three different cosmic reference frames: Is the cosmological principle in jeopardy?

Date: 2014-05-15
Speaker: Dr. Ashok K. Singal, PRL
Venue: Room # 113 (Thaltej Campus)

Abstract

The peculiar velocity of the solar system relative to the CMBR reference frame has been determined to be 369 km/s. However, the velocity relative to the frame of 1.8 million distant radio sources, was found by us to be approximately in the same direction but with a magnitude about 1600 km/s, a result later corroborated by two independent groups. With an aim to hopefully resolve this controversy of two different velocity magnitudes, we have investigated anisotropy in the redshift distribution of distant quasars. For this purpose we have used the latest SDSS catalogue comprising 166583 quasars with reliable accurate redshift measurements. Quite unexpectedly, our results yield the magnitude of the peculiar velocity to be 9750+_550 km/s (~3% the speed of light), ~6 times the NVSS value and as much as a factor of ~25 or so larger than the CMBR value. Even more surprising, the motion is found to be in a direction nearly opposite to that found from CMBR and NVSS. The large differences in the magnitudes of inferred motion as well as their opposite signs are rather disconcerting. A genuine difference between these three velocity vectors would imply highly anisotropic Universe, with anisotropy changing with epoch. This would jeopardize the cosmological principle where the isotropy of the Universe is assumed for all epochs, and without which presently there is hardly any cosmological model.

Chemistry on the Surfaces of Micro and Macro Icy Bodies

Date: 2014-05-08
Speaker: Dr. Bhalamurugan Sivaraman, PRL
Venue: Room # 113 (Thaltej Campus)

Abstract

Condensed molecular species are prevalent in many astrophysical environments including surfaces of dust grains in the interstellar medium, on planetary surfaces in our solar system, as icy aerosols in planetary atmospheres and in comets. These regions are pervaded by ion, photon and electron collisions that are believed to be responsible for the chemistry in synthesizing variety of molecules. Energetic particles alter the molecular composition of these ices and generate new complex species as inferred through the observation of over 180 molecular species in interstellar and circumstellar media. Recent findings from the space and ground based observations on the signatures of several new molecules reveal complex chemistry that is yet least understood under the conditions that are unique to the cold dust in the interstellar medium and icy satellites of our solar system. Such conditions are simulated in the laboratory (in PRL and elsewhere) and the chemistry that underpins such complex molecular synthesis is being explored. In this discussion, we will briefly look into the experimental details along with few results and its implications to the micro and macro icy bodies.

Seeing what we hear: finding electromagnetic counterparts for gravitational wave sources

Date: 2014-05-01
Speaker: Dr. Varun Bhalerao
Venue: Room # 113 (Thaltej Campus)

Abstract

Within the next decade, a global network of advanced gravitational wave detectors including LIGO-India are expected to detect various gravitational wave sources. These detectors can yield certain parameters of the gravitational wave source, but complementary studies in electromagnetic wavelengths are crucial for a complete astrophysical understanding. In this talk, I will talk about the proposed electromagnetic counterparts of these sources, and the extremely challenging problem of detecting them. I will highlight why India forms a key node in this global effort, and discuss prospects of optical, IR, radio and X-ray follow-up from India.

Steady decline in solar polar magnetic fields and heliospheric solar wind micro-turbulence levels: Are we headed towards a Maunder minimum?

Date: 2014-04-24
Speaker: Dr. Susanta K. Bisoi
Venue: Room # 113 (Thaltej Campus)

Abstract

Sunspots or dark regions of strong magnetic fields on the sun are generated via magneto-hydrodynamic processes that involve the cyclic generation of toroidal fields (sunspot fields) from pre-existing poloidal fields and the eventual regeneration of the poloidal fields. This cyclic process, referred to as the solar dynamo, leads to the well known solar activity cycle of waxing and waning sunspot numbers with a period of 11 years. The sunspot minimum at the end of solar cycle 23 was, however, one of the deepest recorded in the past 100 years, with cycle 24 starting about 16 months later than expected. A detailed study of solar high latitude (>45o) or polar magnetic fields using ground based magneto-grams has clearly shown a steady decline in polar field strength since mid-1990's which has continued to the present, i.e. until the end of February 2013. Since sunspot fields are generated from polar fields, this long term decline in polar field strength would eventually effect the sunspot field strength in the subsequent solar cycles. A continued decline in the polar fields in this manner would imply that the polar field strength will approach zero by ~2031. In addition, a detailed analysis of solar wind micro-turbulence in the inner heliosphere has also shown a steady decline in sync with the declining solar photospheric magnetic fields. The fact that both solar polar fields and inner-heliospheric micro-turbulence levels show a similar decline raises the question as to whether we are headed towards an extended period of very little or no sunspot activity in a manner similar to what was seen in the Maunder minimum?

PRL Imaging Polarimeter - observations and first results

Date: 2014-04-10
Speaker: Mr. Ashish Mishra
Venue: Room # 113 (Thaltej Campus)

Abstract

Polarization study in astronomy plays a vital role in exploring astronomical domain starting from our solar system to cosmic beginnings. An imaging polarimeter is being developed in PRL. I will discuss the instrument working principle and the related optics. Data reduction and analysis procedures were developed in python and I will present the algorithms. The calibration of the instrument has been verified for 100% polarization using a glan prism on various stars. Polarization measurements of standard stars are also well matched with those in the literature. Many galactic and extra-galactic sources were observed during a couple of observing runs. For my project I have studied Carbon stars which possess attributes due to which there exists a net intrinsic polarization. Some results will be presented.

Probing the energetics of the multi-wavelength emission from solar flares

Date: 2014-02-13
Speaker: Dr. Arun K. Awasthi
Venue: Room # 113 (Thaltej Campus)

Abstract

Solar flares are known to be gigantic and bursty energy release events occurring in the solar atmosphere. Typically, 10^32 ergs of energy is being released in tens of minutes from a typical solar flare. Defying the standard model of energy release from solar flares, there have been innumerable instances of onset of the precursor heating prior to the commencement of the main phase emission. Although the main phase of energy release in solar flare is studied in greater detail, the underlying processes of energy release causing this precursor emission is in debate. Eventually, observations from current space missions have enabled to undertake extensive study of the precursor phase and their relationship with the whole flare energetics. I will discuss study carried out by me related to the physical processes occurring in the pre-flare plasma and their association to the main phase.

Photometric Study of Short Period Variables

Date: 2014-02-06
Speaker: Mr. Janaka Adassuriya
Venue: Room # 113 (Thaltej Campus)

Abstract

The detail analysis of the photometric data can be used to investigate the short period variability of the stars in the clusters and the individual variables. The present work is based on mainly 50cm CDK telescope data and its detector ANDOR CCD camera which is well capable of detecting very short period variablities in deci maginitude scale. The observations were carried out in three phases during the months of December 2013 and January 2014 at Mount Abu. Several Open Clusters and Delta Scuti type variables were observed in long time scale in different bands for the variablity study. At present the data reduction phase was initiated with some overviews of the observations. The talk is basically coveres the overviews of the observation and some techniques applied to resolve the initial problems. The Point Spred Function (PSF) and Gaussian debelending are very important tools to resolve the crowded fields as Globular Clusters. However the Globular Clusters are the most challanging part of image reduction in astronomy. Relatively Open Clusters are easy to resolve and finding variable candidates. The cluster NGC1513 was observed and instrumental magnitudes are presented. One of the interesting objects observed is SZ Lyn, a delta scuti type variable star with the period of 2.88 hours. The star is observed with a very high time resolution and the differential light curve is presented. The vigilant analyses have to be done in order to make firm conclusions.

Cometary Dust - Probe of Early Solar System

Date: 2014-01-09
Speaker: Prof. U. C. Joshi,
Venue: Room # 113 (Thaltej Campus)

Abstract

The physical and optical properties of cometary dust grains are of great interest at least for two reasons: i) these grains are responsible for much of the light scattered from cometary coma and dust tail; hence grain optics inuences the basic appearances of comets; ii) the grains are supposed to be unprocessed relics from the time of formation of the solar system or before, hence study of cometary grains is very important to investigate physical conditions that existed in the beginning of the formation of solar system. Recent space missions provide substantial and very useful information on the cometary grains. However, physical properties of the cometary dust particles may change from one comet to another. Due to limitation in the number of space missions, ground based observations are important. Solar radiation scattered by cometary dust particles is polarized and hence sensitive to the grain properties, eg, their shape, size distribution and refractive index. Polarimetric observations at dierent phase angles are therefore very useful in understanding the cometary dust properties. We have made photo-polarimetric observations on a large number of comets, e.g., Comet Halley, Hyakutake (C/1996 B2), Hale-Bopp, C/2000 WM1 (LINEAR), etc and more recently, comets 17P/Holmes and C/2007 N3 (Lulin). These observations allowed us to study cometary grains at very low (< 2 degree) to high (~110 degree) phase angles resulting in a wide coverage of negative and positive polarization branches of the polarization-phase curve. Photo-polarimetric observations on Comet Lulin were made when it was close to opposition and we detect a slow linear increase in brightness with decreasing phase angle. Also we nd polarization in negative branch of polarization to be wavelength dependent. The talk will cover a brief introduction and importance of comets and some of the recent results will be discussed.

2013

Scientific photography

Date: 2013-12-19
Speaker: Rakesh Rao
Venue: Room # 113 (Thaltej Campus)

Abstract

Part1: Comet Lovejoy imaging and Landscape Astrophotography The emergence of DSLR cameras as cost effective imagining tool for Astrophotography and landscape astrophotography has revolutionised the way we look at the night sky today. The presentation would showcase the use of DSLR cameras for comet imaging and landscape astrophotography and also basic editing techniques done during the recent observation run at Mt Abu, Rajasthan and few others. Part 2: Journey to the frozen Continent ? Antarctica Situated at the bottom of the Earth, Antarctica - a Continent that expands 14 million square kilometres in area, of which 98 % is covered with snow and ice. Average temperature drop to around -30⁰ Centigrade and in places can reach to as low as 90⁰ Centigrade below zero. Now after 33 years on India?s Presence in Antarctica, India has setup yet another state of art research station ?Bharati? with advance technology to supplement Antarctic research from new location. Journey to Antarctica is a photographic presentation that highlights my 3 consecutive expeditions as a part of the 30th, 31st and the 32nd Indian scientific expedition to Antarctica and emphasises the Indian Antarctica program and the construction of the third Indian research station ?Bharati? in the most hostile condition on this planet. The presentation followed by a short 7 Mins video shall take you through a journey to this bizarre yet amazing Icy continent of Antarctica.

Integral Field Units : Instrumentation for Enabling 3-D Spectroscopy in Astronomy

Date: 2013-12-16
Speaker: Dr. Mudit Srivastava
Venue: Room # 113 (Thaltej Campus)

Abstract

3-D spectroscopy or Integral field spectroscopy (IFS) is a powerful tool for astronomical observations that provides spectra of individual spatial elements over a 2-dimensional area of the sky simultaneously. This in turn allows to reconstruct the image of the observed region at any given wavelength, so IFS enables to perform Hyper-Spectral Imaging of variety of extended objects. At the core of IFS instrumentation is an optical sub system called, Integral Field Unit (IFU). IFUs are the optical interface between the telescope focal plane and the conventional spectrograph, which samples the input field of view into the required spatial elements and, after suitably reformatting, feeds light from each of the spatial elements into the spectrograph that disperses and records the spectra of spatial elements individually. An IFU can be realized using different optical methods to process the incoming light e.g. using tiny lenslets, using optical fibres or using reflective mirrors. In this talk, the speaker will discuss the challenges in the design and development of fibre based method of IFU development and the corresponding IFS data characteristics.

Nera-IR study of fast nova KT Eri

Date: 2013-12-12
Speaker: Dr. Ashish Raj
Venue: Room # 006 (Thaltej Campus)

Abstract

The classical novae are close binary systems with a late-type main sequence star (the secondary) transferring material via Roche lobe overflow to a companion star (the primary). Novae brighten by 10 - 12 magnitudes in a few hours and subsequently fade back to the original faint level over a period lasting several months to years. The results obtained for nova KT Eri from the IR observations will be presented. The case B analysis of hydrogen Brackett series lines has been used to estimate the mass of the gas in the ejecta. We have found the evidence for bipolar flow, which shows the presence of two high velocity components in the nova ejecta. KT Eri is one of the very high latitude nova in last few years with galactic latitude (-32 deg) and distance below the galactic plane z ~ 3.3 kpc.

Study of the inner disk radius during early stages of a black hole outburst

Date: 2013-12-05
Speaker: Anjali Rao
Venue: Room # 113 (Thaltej Campus)

Abstract

Most of the black hole binaries are transient in nature. They are generally in quiescence state and enter into outburst occasionally. The behavior of the inner disk radius of the accretion disk around black holes is not very well studied during early stages of the outburst. We have attempted to study this behavior in 2010 outburst of a black hole candidate MAXI J1659-152 and found that the disk is truncated far from the black hole during its initial hard state. We noticed a systematic infall of the inner disk as the source spectrum becomes softer. In the talk, the results obtained from the study will be discussed.

X-ray and Infrared Properties of Be/X-ray Binary Pulsars

Date: 2013-11-21
Speaker: Dr. Sachindra Naik
Venue: # 113, Thaltej Campus

Abstract

Be/X-ray binaries, consisting of a compact object (neutron star) in orbit around the Be star, form the largest subclass (about 2/3) of High Mass X-ray Binaries (HMXBs). The orbit of the compact object around the Be star is wide and highly eccentric, limiting interactions with the circumstellar disk to when the objects are closest (periastron passage). Be/X-ray binaries are generally quiescent. The transient X-ray outbursts seen in these objects are thought to be due to interactions between the compact object and the circumstellar disk surrounding the Be star. The timing and broad-band X-ray spectral properties of a few Be/X-ray binary pulsars,during the regular X-ray outbursts will be discussed. Along with X-ray properties ofthese accreting pulsar during the outburst, the associated effect on the Be optical companion in infrared and optical bands will also be discussed.

Near-infrared spectral evolution of the 2011 outburst of recurrent nova T Pyxidis

Date: 2013-10-31
Speaker: Dr. Vishal Joshi
Venue: # 113, Thaltej Campus

Abstract

Recurrent Nova is a subclass of nova showing multiple outbursts in human life time. T Pyx is one of the only 11 known nova so far.Near-infrared spectroscopic and photometric observations of the recent 2011 outburst of T Pyx will be presented. Spectra, commencing from one day after outburst, are reported for a total of 18 epochs spread over the 50 days of the eruption. A major finding is the very rare transition of the nova from the He/N to the Fe II class within a few days after the outburst. T Pyx is only second nova to show such transition after V5558 Sgr in the long history of Nova research. The evolution in the strength and shape ofthe line profiles is presented and discussed. A Case B recombination analysis is done which shows that the Brackett lines are a ffected by optical depth e ffects from the start to the end of the observations. Based on the observation that the Br line is clearly optically thick, we are able to place lower limits on the emission measure andelectron density of ejecta.

Declining Solar Activity and the Deepest Solar Minimum in the past 100 years

Date: 2013-10-17
Speaker: Prof. P. Janardhan
Venue: # 113, Thaltej Campus

Abstract

During a roughly 11 year period, the number of sunspots seen on the solar disk shows a cyclic change. The current sunspot cycle (Cycle 24) has been strangely slow to develop and such a small solar maximum has not been observed since theMaunder Minimum from roughly 1645 to 1715. In addition the solar minimum at the end of solar cycle 23 has been one of the deepest in the past 100 years.In this talk, I will discuss the declining solar magnetic activity and look at the signatures of such a decline in the inner-heliosphere.

The Paradox of Power Loss in a Lossless Infinite Transmission Line

Date: 2013-10-03
Speaker: Dr. Ashok K. Singal
Venue: # 113, Thaltej Campus

Abstract

A famous long-standing paradox in Electrical Engineering is of a continuous power drain from the generator at the input of an otherwise lossless infinite transmission line. We show that the resolution of the paradox lies in the realization that in an open-circuit finite line, while the source does keep on supplying power asan incident wave, the reflected wave brings an equal amount back to the source,with no net power loss. The oscillations in even a simple, driven LC circuit are the superposition of incident and reflected waves. But there is no reflected wave in the infinite line, since the incident wave never reaches the termination at infinity to start a reflection. The power lost by thegenerator in the infinite line ultimately appears as the stored electromagnetic energy in the capacitances and inductances further down the line as the incident wave advances forward. It is also shown that contrary to some earlier suggestions in the literature, radiation plays absolutely no role in resolving this paradox.

Chromospheric Activity Induced Barrier in Exoplanet Detection

Date: 2013-09-05
Speaker: Mr. Vaibhav Dixit
Venue: Room # 114, Thaltej Campus

Abstract

Chromosphere is one of the three layers of stellar atmosphere. Stellar activity plays a crucial role in creating unusual temperature rise in the region. The phenomenon has been rigorously studied and modeled for sun. With the advancement of high-resolution spectroscopy, systematic chromospheric studies of various stars have been initiated on a regular basis. Sun-like periodic variations have been observed in most of the stars. In the context of exoplanet detection using radial velocity (RV) method, the chromospheric activity poses a major challenge in detecting low mass planets. With the currently available analytical models it has been predicted that chromospheric activity can induce an artificial RV variations up to 11m/s. While all the major RV survey programs are targeting to reach a meter and sub-meter accuracy, it has become extremely important to take out the activity generated effects on RV values. With the help of PARAS spectrograph we have achieved an RV precision of 1.7m/s for quiet sun like bright stars. The activity models developed from various surveys will help us to break down this barrier and avoid any false detection.

Orbital parameters of an M dwarf arround a hot F0V star in an eclipsing binary system

Date: 2013-08-29
Speaker: Priyanka Chaturvedi
Venue: Room # 113 (Thaltej Campus)

Abstract

Eclipsing Binary systems serve as an effective tool for the precise determination of masses and radii of both the components. Despite a large number of low mass stars present in our galaxy, masses and radii for these stars are still not determined at higher accuracies, primarily due to their fainter magnitudes in visible band. Radii for stars less massive than Sun are known to be 10% smaller theoretically than observed and temperatures are 5% higher. The survey of eclipsing binary candidates from STEREO mission listed an interesting eclipsing binary candidate HD 213597, a hot F0V star, showing box like eclipses with a period of 2.42 days. We have made high resolution spectroscopy measurements on HD 213597 from the HET-HRS spectrograph at McDonald Observatory and PARAS attached to the PRL 1.2m telescope at Mount Abu. In my talk, I will be presenting the results on the orbital parameters like the determination of radial velocity semi amplitude, mass and eccentricity derived from the radial velocity data. I will also be presenting the results from the photometery measurements made on this star from the PRL 1.2 m telescope. Observations of such eclipsing binary systems will statistically contribute to the current limited samples present in the low mass regimes and help us have a better understanding of star formation and stellar evolution theories.

Estimation of the low energy detection limit of a hard X-ray focal plane Compton polarimeter.

Date: 2013-08-01
Speaker: Tanmoy Chattopadhyay
Venue: Room # 113 (Thaltej Campus)

Abstract

Recently, with the advent of hard X-ray mirrors, it is now possible to build a focal plane X-ray Compton polarimeter which will be sensitive up to 80kev. However, the low energy cutoff of the operation range depends on the threshold energy in plastic scintillator used as scatterer. It has been seen that if we can lower down the threshold in plastic, sensitivity of the polarimeter increases significantly. Therefore it is necessary to accurately estimate the low energy detection limit of such plastic scintillators. In this talk, I will explain a method to measure threshold energy experimentally. I will discuss a semi-analytical model that I build up to measure the detection probability in plastic as a function of deposited energies.

Studies of massive star forming regions

Date: 2013-07-18
Speaker: Dr. Watson P. Varricatt,
Venue: Room # 113, Thaltej Campus

Abstract

Low-mass stars are known to form through disk accretion and driving collimated outflows. When it comes to massive stars, the primary mechanism for formation has been a debated topic. The objections to an accretional scenario for the formation came from observational and theoretical considerations, and alternate scenarios like merger of lower mass stars were proposed. We have been observing massive star forming regions using the United Kingdom Infrared Telescope (UKIRT) to understand the main mechanism leading to their formation. In this talk, I will present some of the results obtained from our observations obtained using the UKIRT Wide Field Camera (WFCAM), and from archival data from ground- and space-based facilities. Studies of four interesting massive star forming regions, where multiple star formation is seen to occur in clusters, will be discussed.

Timing and Spectral Properties of High Mass X-ray Binary Pulsars

Date: 2013-07-12
Speaker: Gaurava K. Jaisawal
Venue: Room # 113 (Thaltej Campus)

Abstract

High mass X-ray binaries are interesting objects to study the abrupt mass accretion from the companion star and its effect in the surrounding medium. In case of X-ray pulsars, this abrupt mass accretion changes the geometry of the accretion column near the polar caps. This change in the geometry affects the observed X-ray flux at certain pulse phases which appears as dips in the pulse profiles of the pulsars. These dips in the pulse profiles are found to be strongly energy dependent. Pulse phase resolved spectroscopy confirmed the presence of additional matter at certain pulse phases giving rise to dips in the pulse profiles. Broad-band spectroscopy of these pulsars also helps us to investigate the properties of these objects. In the seminar, I will discuss the timing and spectral studies of a few binary X-ray pulsars to understand their properties.

Tau Ceti and the search for Exo-earths around bright G,K dwarfs

Date: 2013-06-27
Speaker: Dr. Abhijit Chakraborty
Venue: Room # 113 (Thaltej Campus)

Abstract

Tau Ceti is only 11.9 light years away, it is the second-closest single G-type star (G9V). Its older than the Sun about 6 Gyrs, about one-third in metalicity, photometrically quiet and a radial velocity standard star. Tau Ceti is so bright and so quiet in terms of stellar jitter (seismology) that it is among the handful of stars in the sky that are best suited to long-term Doppler high-precision monitoring and defines the spectrograph stability. The star is visible in both the hemisphere and so it is in the list of all of the major Doppler surveys. In 2004 a huge cold Debris Disk with about the ten times the mass of the Kupier Belt was detected at Pluto-like distance from the star. The detection was in sub-mm wavelength using JCMT. A hypothesis was put forward that the debris disk could indicate presence of Rocky bodies from a few 100 meters size onwards and perhaps presence of inner Earth size planets. HARPS (at 1.5m/s RV precision) did a long term survey of 5 years up to 2011 and found no evidence of planet at a level of 1.5m/s precision (a few Earth Mass). Subsequently, in late 2012 a different data analysis approach on the same HARPS data was applied by another group of Astronomers and this time authors found evidence of 5 planets with one of them possibly in the habitable zone. Although this new result is highly debated in the last six months, it nevertheless opens up the necessity of further high precision continuous monitoring of the star. Finally, I will talk about PARAS efforts from Mt. Abu. From Feb 2013 we have began observing tau Ceti with PARAS and got short term precision of 1.3m/s which is very competitive with respect to HARPS. For us this serves the dual purpose of proving Spectrograph stability and as well as the Science. I will also discuss other bright G,K dwarfs with low stellar jitter like sigma Draconis (G9V). We have monitored sigma Draconis for about a year at 1.8m/s RV precision. The star has some similar characteristics with respect to tau Ceti. Although, we have not detected any earth-planet around sigma Draconis so far, it establishes PARAS capability to do so and long term stability. I will also discuss about possible planet mass upper limits on sigma Draconis. Other exo-Earths potential candidates that we have began observing recently are HD 9407 (G6V) and HD166620 (K3V).

Are radio morphology and optical luminosity of a radio galaxy really related?

Date: 2013-06-20
Speaker: Dr. Ashok K. Singal
Venue: Room # 113 (Thaltej Campus)

Abstract

One of the robust correlations in all observational astronomy is between the morphology type of radio galaxies and their radio luminosity. First pointed out by Fanaroff and Riley in 1974, there is a very sharp dependence of the morphology types of radio galaxies on their luminosities, so that almost all radio galaxies below a critical "break-luminosity" are edge-darkened (called FR I), while all others are edge-brightened (called FR II). This correlation has withstood the test of time. Since early 1990s there are many reports in the literature of the break-luminosity correlated with the absolute visual magnitude of the host galaxy. Based on these observational results there are many theoretical models trying to explain the dependence of radio morphology on optical luminosity, and this medelling business has grown into quite an industry. However we show that this observed correlation actually arises because of some subtle selection effects and is therefore not an intrinsic property of the radio galaxy population. It looks like all such models may have to be abandoned, unless otherwise one finds some other independent observational evidence of correlation between radio morphologies and optical luminosities.

Is there a violation of the Copernican principle in radio sky?

Date: 2013-06-06
Speaker: Dr. Ashok K. Singal
Venue: Room # 113 (Thaltej Campus)

Abstract

Cosmic Microwave Background Radiation (CMBR) observations from the WMAP satellite have been reported in recent years to show some unexpected anisotropies, which surprisingly seem to be aligned with the ecliptic. This alignment has been dubbed the ``axis of evil'' with very damaging implications for the standard model of cosmology. The latest data from the Planck satellite have confirmed the presence of these anisotropies. We have found even larger anisotropies in the sky distributions of powerful extended quasars and some other sub-classes of radio galaxies in the 3CRR catalogue, one of the oldest and most intensively studies sample of strong radio sources. The anisotropies lie about a plane passing through the two equinoxes and the north celestial pole (NCP). We can rule out at a 99.995 percent confidence level the hypothesis that these asymmetries are merely due to statistical fluctuations. Two pertinent questions that arise here are - firstly why should there be such large anisotropies present in the sky distribution of some of the most distant discrete sources implying inhomogeneities in the universe at very large scales (covering a fraction of the universe)? Secondly why should such anisotropies lie about a great circle decided purely by the orientation of earth's rotation axis and/or the axis of its revolution around the sun? It looks as if these axes have a preferential placement in the larger scheme of things, implying an apparent breakdown of the Copernican principle or its more generalization, cosmological principle, upon which the standard cosmological model is based upon. Has ghost of Aristotel struck with vengeance?

Understanding the long timescale variability in blazars, a case of FSRQ PKS 1510-089

Date: 2013-05-23
Speaker: Sunil Chandra
Venue: Room # 113 (Thaltej Campus)

Abstract

BL Lacs and Flat Spectrum Radio Quasars (FSRQs) are collectively known as Blazars. It is well established fact blazars are a subclass of Active Galactic Nuclei (AGNs) seen at very small angles (<=15 deg) to the jet of relativistic plasma emanating perpendicular to the plane of accretion disk. Blazars emit non-thermal continuum emission over the entire energy spectrum i.e ranging from radio to VHE Gamma-Rays through Optical/X-ray. The main differences between BL Lacs and FSRQ are, to first order, that the FSRQ are more distant, more luminous, and have stronger emission lines. This era can be assumed to be the golden time for studying such objects. A rich sample of blazars are regularly monitored by VLBI radio and several optical facilities all over the globe. Swift/AGILE/SUZAKU/MAXI and Fermi are providing good amount of data at high energies. HESS/MAGIC and several others are exploring Extreme Energy Gamma-Rays (TeV) which completely untouched part of spectrum . In short we have opportunities to get the complete coverage of blazar SED almost simultaneously. There are several studies by many authors about disk and jet connections in blazars. Though the formation and collimation of jets in AGN are not well understood, one can estimate some of the key properties of the system based on present observations. In the present study we are investigating FSRQ PKS 1510-089 for variability of all possible time scales in different energy bands. The purpose of this seminar is to review the current understanding these systems and here we are using PKS 1510-089 as an example.

X-ray binary evolution and their collective properties

Date: 2013-05-16
Speaker: Dr. Harshal Bhadkamkar
Venue: Room # 113 (Thaltej Campus)

Abstract

X-ray binaries are the binary systems of a normal star and a compact object. Compact object accretes material from the normal star and the accretion energy is observed as X-ray emission from these sources. X-ray binaries are considered as one of the dominant X-ray sources in galaxies and their collective emission is thought to be a major part of the total X-ray luminosity of galaxies. I will first briefly discuss the life-history of these objects. I will then go on to show how are the evolutionary mechanisms used to derive the collective properties of these objects. Theoretical results derived will be compared with the observations. I will then turn to the subject of tidal evolution in these objects. I will discuss the effects of tidal synchronization on the orbit and subsequently on the evolutionary path. Finally I will present future outlook, looking ahead of the simplified evolutionary picture.

A study on sudden impulses in the geo-magnetic fields

Date: 2013-04-25
Speaker: Susanta Kumar Bisoi
Venue: Room # 113 (Thaltej Campus)

Abstract

Solar initiated interplanetary disturbances, the direct consequences of explosive events on the Sun such as Coronal Mass Ejections (CMEs) and solar flares, drive space weather events and are the obvious sources for geo-magnetic events on the earth. In this talk, I will begin with a discussion on the coupling of the solar wind plasma with the earths magnetosphere and their subsequent role in development of different magnetospheric and ionospheric processes that prompt or trigger the buildup of geo-magnetic events. In addition, our recent work on the study of sudden impulses in the geomagnetic fields will be discussed.

Noise reduction in lunar occultation light curves using Wavelet and Fourier transforms.

Date: 2013-04-04
Speaker: Dr. Tapas Baug
Venue: Room # 114 (Thaltej Campus)

Abstract

Lunar Occultation (LO) is a powerful ground based technique to measure the angular diameter of stars even with a moderate size 1 m class telescope. A good signal to noise ratio (S/N ~ 40) of the observed light curve is prime requirement to extract proper angular diameter information of the source. However, an LO light curve with better S/N can also help to measure the circumstellar contribution of a star, in addition to its stellar angular diameter. With present set of instruments 2-magnitude fainter objects are being observed. Hence, methods to improve S/N of the observed light curve using Wavelet Transforms and Fourier Transforms have been studied. In previous talk, the aspect of S/N improvement was discussed. Further analysis has been carried out for S/N enhancement of the light curves, as well as, to examine the improvement in their model fits. Particulars will be presented.

A tyro's foray into mathematics -- my solutions of the cubic equation

Date: 2013-03-21
Speaker: Prof. D.P.K. Banerjee
Venue: Room # 114 (Thaltej Campus)

Abstract

The history of algebraic equations has oodles of high voltage drama in it. The cubic equation was first solved by a stammering Tartaglia, a man who had his face slashed by a sword. It also involves a thieving mathematician (Cardano) who stole those first results and passed them off as his own. The fifth order equation, the quintic, is inseparably linked with the tragic genius of Evariste Galois killed in a duel at the impossibly young age of 20 ; much younger in death than Byron or Shelley or Keats. My foray into this world was to understand why the standard method of solving the cubic equation always worked with the "reduced" form of the equation. Was there not a way in which one used the entire equation? In this talk I will answer this question, offer new solutions and also present an unified method to solve algebraic equations. I will also explain how solving crosswords proved to me that the human brain could work in its sleep and validated the inscrutable advice by one of PRL's most distinguished scientists to a young researcher "Don't waste time in the day in thinking, that can be done at night when you sleep".

Blazar sequence and polarization

Date: 2013-02-14
Speaker: Prof. K. S. Baliyan
Venue: Room # 114 (Thaltej Campus)

Abstract

Blazars are powerful AGNs with emission dominated by the relativistic jet. Their continuum spectral energy distribution is characterized by two humps, peaking at low and high energies. Based on the position of lower peak (called synchrotron peak) and other properties, blazars are classified into a sequence, called blazar sequence; FSRQ, LBL, HBL. Their luminosity decreases from FSRQ to HBL and so does the gamma-ray emission dominance. These properties help in the study of a blazar candidate. It is also noticed that generally FSRQs have higher degree of polarization as compared to HBLs and we show that polarization can be used to classify blazars. For this, polarization and spectral energy distribution of a blazar candidate will be discussed.

Demise of the Grand Unified Scheme of Powerful Radio Sources

Date: 2013-01-31
Speaker: Dr. Ashok K. Singal
Venue: Room # 114 (Thaltej Campus)

Abstract

In the currently popular orientation-based unified scheme (OUS), a radio galaxy appears as a quasar when its principal radio-axis happens to be oriented within a certain cone opening angle around the observer's line of sight. Due to geometrical projection, the observed sizes of quasars should therefore appear smaller than those of radio galaxies. We show that this simple, unambiguous prediction of the unified scheme is not borne out by the actually observed angular sizes of radio galaxies and quasars. Thus OUS is ousted. It seems that no straightforward modification, including an evolution of source properties with luminosity or cosmic epoch, could within the current model make it compatible with observations, and to resolve the dichotomy of radio galaxies and quasars, a scheme totally different from the present would be required.

Energy release processes during precursor and main phase emission in Solar flare

Date: 2013-01-24
Speaker: Arun K. Awasthi
Venue: Room # 114 (Thaltej Campus)

Abstract

The time evolution of energy release in solar flare is categorized in three phases viz: precursor, impulsive and gradual. The impulsive and gradual phase of energy release in solar flare are studied in greater detail but the underlying processes of energy release and their relationship with the precursor phase is in debate. The high spatial, spectral and temporal resolution observations are the constraint in this investigation. Observations from current space mission enable to undertake extensive study of the precursor phase and relationship with the whole flare energetics. I will discuss study carried out by me related to the physical processes occurring in the pre-flare plasma responsible for triggering the main phase of energy release in solar flares and propose the model to improve the understanding of the relationship between precursor and main phase energetics.

A study of North-South asymmetry of sunspot areas during cycle 23 and 24

Date: 2013-01-17
Speaker: Dr. Partha Chowdhury,
Venue: Room # 114 (Thaltej Campus)

Abstract

Sunspots are strongly magnetized regions on the solar surface and are the seats of solar magnetic storms like solar flares and coronal mass ejections – that eject large amounts of magnetized plasma into the space generating hazardous effects to satellites and modern day technologies. Although solar activity appears reasonably symmetric in the two hemispheres after short-term variations are averaged, some cycles have been found to be stronger in one hemisphere. The N-S asymmetry is important for the topology of the interplanetary space, and it influences both interplanetary and near-Earth space parameters. An understanding of hemispheric variation of solar activity may provide insight into the complex dynamic behavior of the solar dynamo in the two hemispheres. In the present work, we study the statistical significance of north–south asymmetry of sunspot areas for the complete cycle 23 and the ascending phase of current cycle 24. The preferred hemisphere in each year of cycle 23 and 24 has been identified. The statistically significant intermediate-term periodicities of the north-south asymmetry of sunspot data have also been investigated. Power spectrum analysis have shown a number of short and mid-term periods including the Rieger type in both cycles 23 and 24.We have also investigated the present strength of current cycle 24 and a comparative study has been done about different previous predictions regarding the strength of this cycle. We present our results and discuss their possible explanations with the help of theoretical models and previous results.

The structure and evolution of protoplanetary disks : Spitzer IRS survey of young stars in the nearby star forming regions

Date: 2013-01-15
Speaker: Dr. Puravankara Manoj
Venue: Room # 114 (Thaltej Campus)

Abstract

Planetary systems are thought to be formed out of protoplanetary disks surrounding young stars. We have obtained 5- 40 micron mid-infared spectra of some 600 protoplanetary disks around young solar-mass stars in the nearby star forming regions of Taurus-Auriga, Chamaeleon I, Ophiuchus and Orion A, with the infrared spectrograph (IRS) on board the Spitzer space telescope. The shape of the mid-infrared continuum and the dust emission features seen in the IRS spectra of such a large sample allow us to study the structure of protoplanetary disks and disk evolutionary indicators that mark the onset of planet formation process such as dust grain growth, sedimentation and crystallization in unprecedented detail. I will describe our analysis and present our main results. I will discuss the implications of these results and the new insights that they provide on our understanding of disk evolution and planet formation.

Chromospheric Magnetic Field of the Sun

Date: 2013-01-09
Speaker: Prof. Debi Prasad Choudhary
Venue: Room # 114 (Thaltej Campus)

Abstract

After one hundred year of discovery of magnetic field in sunspots, several outstanding questions related to the origin of explosive events at these locations remain unanswered. In the mean time, many sun like stars have have been found to display super flares, understanding of which need an intensive study of solar phenomena. The key to further advance the field of solar explosion science is observational study of three dimensional magnetic field configuration of active regions. While, in the past measurements mostly focused magnetic field in the photospheric heights, many future instruments plan chromospheric observations. In this presentation, we shall review the current understanding of solar chromospheric magnetic field.

Three-dimensional Structure of Inner Heliosphere during Solar Cycles 22-24

Date: 2013-01-07
Speaker: Prof. P. K. Manoharan
Venue: Room # 114 (Thaltej Campus)

Abstract

The inner heliosphere, which can be defined as the region of space beyond the solar corona and out to ~5 AU, is highly structured on all observable scales. Moreover, the large-scale ambient structure of the inner heliosphere is dominated by interactions between ambient solar wind and (i) high-speed streams from coronal holes (e.g., corotating interaction regions, CIRs) (ii) coronal mass ejections (CMEs). This knowledge is based on the interpretation of a wide array of remote and in situ measurements. In this talk, I will review our current knowledge of the three-dimensional structure of the quasi-steady, large-scale inner heliosphere during solar cycles 22 24. A downward trend in density turbulence has been observed from the maximum of cycle 22 to the deep minimum phase of cycle 23 and beyond. Moreover, the latitudinal distribution of solar wind speed has been significantly different during the above period. I will highlight the properties of the peculiar solar minimum, which has been, thus far, quite distinct.

Disks and Outflows around Massive YSOs

Date: 2013-01-03
Speaker: Dr. Bhargav Vaidya,
Venue: Room # 114, Thaltej Campus

Abstract

The inner most regions around massive young stellar objects (YSO) are associated with complex interactions between numerous physical processes. Since the inner few Astronomical Units (AU) are tough to resolve observationally, a theoretical approach is important to create a qualitative picture for these regions around young high-mass stars. In this talk, I will focus on the interplay between important physical processes with respect to the dynamics of jets and inner accretion disks. In particular, I will discuss the applicability of a thin accretion disk model with proper dust and gas opacity for a luminous young high-mass star. Further, I will examine the stability of such inner accretion disks and argue that they form an ideal launching base for long-lasting outflows. Outflows and jets are an ubiquitous phenomenon in young massive star forming regions. Observational surveys have suggested that the outflows become wider as the star grows in luminosity (thus mass) with time. In this talk, I will present results from magneto-hydrodynamic (MHD) simulations of wind launching in presence of radiative forces from the luminous star and the inner hot accretion disk. The major outcome of this work, is that the radiative force from the central star plays a dominating role in accelerating and de-collimating the magnetically launched jet, while the influence of the disk radiative force is rather small. This interplay of radiative and magnetic forces provides a physical insight to the trend in degree of collimation suggested by observations. Lastly, I will apply this model of MHD wind driving to study a particular outflow from a

2012

Solar Thermal Applications

Date: 2012-12-05
Speaker: Dr. N. M. Nahar
Venue: Room # 114, Thaltej Campus

Abstract

Energy-dependent thermal emission in solar flares

Date: 2012-10-04
Speaker: Mr. Arun Kumar Awasthi
Venue: Room # 114, Thaltej Campus

Abstract

Is Interstellar Space Travel Possible?

Date: 2012-09-27
Speaker: Dr. Ashok K. Singal
Venue: Room # 114, Thaltej Campus

Abstract

Radiation pressure instability in unstable disks showing 'heartbeats'

Date: 2012-09-21
Speaker: Ms. Anjali Rao
Venue: Room # 114, Thaltej Campus

Abstract

A first-time detection of giant pulses from a normal-period pulsar B0950+08

Date: 2012-08-16
Speaker: Dr. Ashok K. Singal
Venue: Room # 114, Thaltej Campus

Abstract

Correlated Switching in the Radio and X-ray Emission Properties of Pulsar B0943+10: Evidence for Rapid, Global, Magnetospheric Changes revealed by XMM-Newton, GMRT and LOFAR

Date: 2012-07-23
Speaker: Prof. Willem Hermsen
Venue: Room # 114, Thaltej Campus

Abstract

Why IGR J17091-3624 is so faint? Constraints on distance, mass and spin from phase-resolved spectroscopy of 'heartbeat oscillations'

Date: 2012-07-23
Speaker: Ms. Anjali Rao
Venue: Room # 114, Thaltej Campus

Abstract

Probing the Brown Dwarf Desert

Date: 2012-07-05
Speaker: Ms. Priyanka Chaturvedi
Venue: Room # 114, Thaltej Campus

Abstract

Compton polarimeter as a focal plane detector for hard X-ray telescope: Sensitivity estimation with Geant4 simulations

Date: 2012-06-26
Speaker: Mr. Tanmoy Chattopadhyay
Venue: Room # 114, Thaltej Campus

Abstract

Variability in Blazars

Date: 2012-06-07
Speaker: Mr. Sunil Chandra
Venue: Room # 114, Thaltej Campus

Abstract

The Nuclear Spectroscopic Telescope Array (NuSTAR)

Date: 2012-06-04
Speaker: Dr. Vikram R. Rana,
Venue: Room # 114, Thaltej Campus

Abstract

Gyro-orbit size and the implausibility of coherent emission by bunching in synchrotron radio sources

Date: 2012-05-24
Speaker: Dr. Ashok K. Singal
Venue: Room # 114, Thaltej Campus

Abstract

A study of the role of Lyman beta fluorescence on OI line strengths in Be stars

Date: 2012-05-17
Speaker: Dr. Blesson Mathew
Venue: Room # 114, Thaltej Campus

Abstract

Energy-Dependent Thermal Emission and Cooling in Solar Flare Plasma

Date: 2012-05-10
Speaker: Mr. Arun Kumar Awasthi
Venue: Room # 114, Thaltej Campus

Abstract

Asymmetry in periodicities of photospheric solar magnetic fields and its implications

Date: 2012-04-26
Speaker: Mr. Susanta Kumar Bisoi
Venue: Room # 114, Thaltej Campus

Abstract

Is the Precursor phase a cause to trigger the main phase energy release in Solar flares?

Date: 2012-04-19
Speaker: Mr. Arun Kumar Awasthi
Venue: Room # 114, Thaltej Campus

Abstract

The build-up to the deep solar minimum between solar cycles 23-24: Earliest inner heliospheric signatures

Date: 2012-04-12
Speaker: Mr. Susanta Kumar Bisoi
Venue: Room # 114, Thaltej Campus

Abstract

Multi-wavelength Variability of Blazars

Date: 2012-04-03
Speaker: Dr. Alok Gupta
Venue: Room # 114, Thaltej Campus

Abstract

The efforts to improve the signal to noise ratio of Lunar Occultation light curves using Wavelet Transforms and Fourier Transforms

Date: 2012-03-29
Speaker: Mr. Tapas Baug
Venue: Room # 114, Thaltej Campus

Abstract

Current trends and some Examples of CCD and IR Imaging systems

Date: 2012-03-06
Speaker: Prof. Robert Leach
Venue: Room # 114, Thaltej Campus

Abstract

2011

Some Null-Impedance (Zero-Resistance) Paradoxes in Electrical Circuit Theory

Date: 2011-12-15
Speaker: Dr. Ashok K. Singal
Venue: Room # 114, Thaltej Campus

Abstract

The outburst and nature of young eruptive low mass stars in the dark clouds

Date: 2011-09-15
Speaker: Dr. Devendra Ojha
Venue: Room # 114, Thaltej Campus

Abstract

Gas and Dust in the Magellanic Clouds

Date: 2011-09-08
Speaker: Dr. Ananta C. Pradhan
Venue: Room # 114, Thaltej Campus

Abstract

Near Infrared Studies of Nova V5579 Sgr

Date: 2011-08-11
Speaker: Mr. Ashish Raj
Venue: Room # 114, Thaltej Campus

Abstract

Lunar Occultation of a few AGB Stars from Mt. Abu Observatory

Date: 2011-08-04
Speaker: Mr. Tapas Baug
Venue: Room # 114, Thaltej Campus

Abstract

Peculiar motion of the solar system from the sky brightness anisotropy

Date: 2011-07-11
Speaker: Ashok K. Singal,
Venue: Room # 114, Thaltej Campus