Astronomy & Astrophysics Seminars

Active Galactic Nuclei (AGN) are powered by accretion onto supermassive black holes and, in the case of blazars, exhibit emission dominated by relativistic jets closely aligned with our line of sight. In the classical orientation-based unification scheme, type-1 AGN, type-2 AGN, and blazars are regarded as fundamentally similar systems viewed at different angles. However, the growing discovery of AGN undergoing intrinsic state transitions on human timescales challenges this static framework and points to dynamical evolution within the central engine. 

In this seminar, I will present results addressing both the long-term persistence and rapid transitions of the blazar state in beamed radio quasars. Using high-quality optical light-curves from the Zwicky Transient Facility (ZTF) survey and the polarization measurements from the RoboPol survey, we investigated the persistence of the blazar state in individual radio quasars. We find that ~90% of beamed radio quasars retain their blazar mode over 3–4 decades, although transitions on year-like timescales can also occur, likely associated with short-lived jet events. Complementary systematic intranight optical variability (INOV) studies of 14 high-redshift blazars (FSRQs) provide the first characterization of rest-frame UV intranight variability, suggesting that UV synchrotron emission may arise from a particle population distinct from that producing up to near-infrared/optical emission. Extending this analysis to low-mass AGN (MBH ~10⁶ M⊙), we detect blazar-like activity, implying that relativistic jets can operate even in substantially lower-mass systems.

Finally, I will discuss rare transition objects, including the radio-state transition quasar J0950+5128 and the changing-look blazar OQ 334, which serve as natural laboratories for probing the onset and evolution of jet activity and its connection to accretion processes. Together, these results establish optical variability as a powerful probe of AGN state evolution and jet–accretion coupling.

In this talk, I will explore two complementary views on the evolution of the Milky Way Galaxy. In the first part, I will present a chemo-dynamical analysis of stars from the APOGEE survey within 5 kpc of the Sun. We separate these stars into thin disk, thick disk, innerhalo, and outer halo populations based on their orbital properties. We find systematic metallicity gradients within these populations. The inner halo has more α-elements, and we also see substantial trends with orbital radius and eccentricity. These results show how chemical enrichment, dynamical heating, radial migration, and accretion all work together to shape the Galaxy as it is today. In the second part, I will turn to the nucleosynthetic origin of trans-iron elements in carbon-enhanced metal-poor stars enriched with both s- and r-process elements (CEMP-rs). Despite decades of progress, the astrophysical sources of these elements remain uncertain. These elements are thought to be produced by a variety of nucleosynthetic processes, the main ones being the so-called slow (s) and rapid (r) neutron capture processes. An intermediate neutron capture process (i-process) is also thought to occur at neutron densities intermediate between the s- and r-processes. We report the discovery of tantalum, a rare third r-process peak element and a powerful diagnostic of i-process nucleosynthesis, in CEMP-rs stars. The pattern of its abundance provides strong evidence for i-process enrichment, giving us new clues about where trans-iron elements come from. Together, these studies connect the Milky Way’s structure and motion with unusual ways of making heavy elements, helping us better understand how our Galaxy formed and acquired its chemical makeup.

Black holes of masses over a million times our Sun, inhabit the centres of most galaxies and appear to co-evolve with their host galaxies. While we do not yet fully understand how galaxies and their central supermassive black holes grow hand in hand, evidence suggests that such black holes play a significant role in regulating galaxy assembly. We are able to spot these black holes to the far reaches of the universe if and when they accrete matter, which is also what causes them to impact their environments out to spatial scales that are well beyond their gravitational sphere of influence. In this talk I will discuss the understanding that has emerged from studies of the systematics of such accreting supermassive black holes from across the electromagnetic spectrum, and also pointers to the way forward.

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.

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.

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.

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.