SEMINAR

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

Date : 21-03-2024
Time : 16:00:00
Speaker : Dr. Alexandre SANTERNE
Area : Astronomy & Astrophysics Division
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.

Title : A Multi-wavelength Study of Magnetic Cataclysmic Variables

Date : 20-03-2024
Time : 16:00:00
Speaker : Nikita Rawat
Area : Astronomy & Astrophysics Division
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.

Title : Probing the Hidden Universe: Direct and Indirect Searches of Dark Matter

Date : 19-03-2024
Time : 14:30:00
Speaker : Dr. Divya Sachdeva
Area : Theoretical Physics
Venue : Online ---> https://imeet.vconsol.com/join/9022656493?be_auth=NTAzNzE4

Abstract

Recent years have seen strong support for dark matter (DM) from observations on a multitude of scales, with a range of experiments operational, approved, or proposed to explore DM theories across various masses. The vastness of the parameter space of DM necessitates exploring model-dependent and model-independent approaches while investigating the symbiosis between astrophysical observations and particle physics experiments. In this talk, I will highlight my past and present research projects where we obtained limits on parameter space of various DM candidates and outline future research plans. I will conclude with a discussion on two key areas of my research plan: a) the impact of potential interaction of DM with Standard Model (SM) in altering its velocity distribution and expanding the detectable parameter space for direct detection experiments; and b) the presence of non-relativistic DM near supermassive black holes, which can lead to high-density DM regions that produce robust annihilation signals, thereby enhancing our indirect search capabilities.

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

Date : 19-03-2024
Time : 16:00:00
Speaker : Vineet Rawat
Area : Astronomy & Astrophysics Division
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.

Title : Hearing the Universe Hum with Gravitational Waves and Primordial Black Holes at Pulsar Timing Array: astrophysical, cosmological and particle physics interpretations

Date : 18-03-2024
Time : 16:00:00
Speaker : Dr. Anish Ghoshal
Area : Theoretical Physics
Venue : Room no: 469 (Main Campus)

Abstract

We will discuss interpretation of the nHz stochastic gravitational wave background (SGWB) seen by NANOGrav and other Pulsar Timing Array (PTA) Collaborations in the context of supermassive black hole (SMBH) binaries. The frequency spectrum of this stochastic background is predicted more precisely than its amplitude. We will discuss how Dark Matter friction can suppress the spectrum around nHz frequencies, where it is measured, allowing robust and significant bounds on the Dark Matter density, which, in turn, controls indirect detection signals from galactic centers. Next we will discuss alternative cosmological interpretations including cosmic strings, phase transitions, domain walls, primordial fluctuations and axion-like physics each of which may lead to Primordial Black Hole formation. Focussing on primordial black holes (PBHs) production in various cosmological scenarios involving single-field inflation, multiple fields, particularly the Curvaton model, as well as those based on the presence of remnants dominated by the false vacuum and show the PBH formation from these remnants including the contribution from the false vacuum and the bubble walls, during strong first-order phase transition by estimating the collapse using the hoop conjecture. Such PBH formations have associated Gravitational Waves from bubble collisions, the spectral shape of which is distinct from that of scalar-induced GW. Finally we will end by putting these comparative studies to test via We will discuss how well these different hypotheses fit the NANOGrav data, both in isolation and in combination with SMBH binaries, and address the questions: which interpretations fit the data best, and which are disfavoured. Finally we also discuss experimental signatures that can help discriminate between different sources of the PTA GW signals with complementary probes using CMB experiments and searches for light particles in DUNE, IceCUBE-Gen2, neutrinoless double beta decay, and forward physics facilities at the LHC like FASER nu, etc. along with Primordial Black Hole formation and its constraints.

Title : Volcanism along the rings of the crisium basin on the moon date

Date : 15-03-2024
Time : 16:00:00
Speaker : Ms. Neha Panwar
Area : Planetary Sciences Division
Venue : Seminar Room # 113/114 (Thaltej Campus)

Abstract

The Crisium Basin (17.0 °N, 59.1 °E) is a ~1100 km multi-ring basin formed at ~4.07Ga hosting widespread volcanism within its central depression and along its rings. The results from Luna 24 and remote sensing observations suggest that the basalts of the Mare Crisium have been emplaced between 3.45 to 2.52 Ga. The basalts from the Inner Depression display a range of composition as indicated by their TiO2 abundances that vary from 6-10 wt.% in the east to <2 wt.% in the west. The sub-surface units exposed by larger craters are also compositionally distinct from Mare Crisium basalts, thereby revealing the possibility that the composition of the basalts evolved with time. However, there has been no comprehensive study regarding the composition and emplacement timescales of the basalts along rings of the Crisium Basin. The basalts along the rings of the Crisium Basin have been emplaced within Mare Undarum, Mare Spumans, Mare Anguis, Cleomedes Crater and Lacus Bonitatis. My recent study identified Marginis West as an episode of volcanism along the outermost ring of the Crisium Basin. This study, for the first time, examines the compositional diversity and ages of the basalts emplaced along the rings of the Crisium Basin to better understand its geological evolution. In the seminar, the results will be discussed.

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

Date : 14-03-2024
Time : 16:00:00
Speaker : Birendra Chhotaray
Area : Astronomy & Astrophysics Division
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.

Title : Changing-State AGNs: Challenging our Understanding of AGNs

Date : 13-03-2024
Time : 11:00:00
Speaker : Dr. Arghajit
Area : Astronomy & Astrophysics Division
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.

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

Date : 11-03-2024
Time : 16:00:00
Speaker : Dr. Mayukh Pahari
Area : Astronomy & Astrophysics Division
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.