Professor Gulab Dewangan (Inter-University Centre for Astronomy and Astrophysics, India)
Abstract: Nearly all galaxies harbor supermassive black holes at their cores. In a subset of these galaxies, the accretion of matter onto the central supermassive black hole generates substantial radiation across almost the entire electromagnetic spectrum. These astrophysical objects, known as active galactic nuclei (AGN), are typically persistent and variable sources, serving as cosmic laboratories for studying various physical phenomena, including the effects of strong gravity near the central black hole. Recent observations have unveiled a fascinating category of galactic nuclei that exhibit extreme variability, such as very large amplitude quasi-periodic X-ray eruptions (QPEs), complete transformations in spectral appearance, referred to as changing-look AGNs. Additionally, the tidal disruption of stars by central supermassive black holes can trigger accretion, leading to transient phenomena in otherwise quiescent galactic nuclei. Multi-wavelength observations of these highly variable events and transient phenomena in galactic nuclei are providing an abundance of information, offering new insights and posing significant challenges. The speaker will present an overview of galactic nuclear activity and discuss recent findings, particularly those derived from India's pioneering multi-wavelength space observatory, AstroSat.
