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.
Dr. Bart Ripperda (Canadian Institute for Theoretical Astrophysics (CITA), University of Toronto, Canada)
Astrophysical black holes are surrounded by accretion disks, jets, magnetospheres, and coronae consisting of magnetized relativistic plasma. They produce observable multi wavelength and multi messenger signals from near the event horizon and it is currently unclear how this emission is exactly produced. The electromagnetic radiation typically has a non-thermal component, implying a power-law distribution of emitting relativistic electrons. Magnetic reconnection and plasma turbulence are viable mechanisms to tap the large reservoir of magnetic energy in these systems and accelerate electrons to extreme energies. The accelerated electrons can then emit high-energy photons that themselves may strongly interact with the plasma, rendering a highly nonlinear system. In some cases the electromagnetic emission is accompanied by a multi messenger signal in the form of neutrinos, cosmic rays, or gravitational waves. Modeling the emitting systems necessitates a combination of magnetohydrodynamic models to capture the global dynamics of the formation of dissipation regions, and a kinetic treatment of plasma processes that are responsible for particle acceleration, quantum electrodynamics effects like pair creation and annihilation, and radiation. I will present novel studies of accreting black holes and how they radiate in regions close to black hole event horizon, using both first-principles general relativistic kinetic particle-in-cell simulations and global large-scale three-dimensional magnetohydrodynamics models. With a combination of models, I determine where and how dissipation of magnetic energy occurs, what kind of emission signatures are typically produced, and what they can teach us about the nature of black holes.
Gopal Bhatta (Institute of Astronomy, University of Zielona Góra)
X-ray emission from blazars provides crucial insights into the underlying AGN processes, as it can arise from synchrotron and inverse-Compton mechanisms in leptonic models, or be part of particle cascades produced in hadronic models. Here we present multi-instrument, multi-epoch investigations of several prominent blazars (OJ 287, PKS 2155–304, 3C 273, and Mrk 421 and samples of other blazars) with a particular focus on their X-ray variability and spectral properties. We perform comprehensive timing and spectral analyses covering intraday to decadal timescales, using long-term observations from Swift, XMM-Newton, NICER, and NuSTAR spanning more than two decades. During the period, the sources consistently display strong variability, with X-ray flux changes reaching factors of ~10–200 during major flares. Timing analysis, including fractional variability, flux and rms distributions, PSD slopes, and rms–flux relations, reveal a combination of normal, lognormal, and bimodal flux states, indicating complex, multiplicative variability processes and transitions between distinct emission states. Spectral analyses using power-law, log-parabola, and broken power-law models demonstrate that log-parabolic or broken spectra best represent the majority of observations, pointing to curvature in the underlying electron energy distributions. Several sources show clear flux-dependent spectral evolution, including “harder-when-brighter’’ behavior and strong correlations between synchrotron peak energy, curvature parameters, and X-ray flux. In OJ 287, broad-band spectral energy distribution (SED) modeling highlights systematic transitions between inverse-Compton–dominated quiescent states and synchrotron-dominated flaring states. Collectively, the observed properties such as Gaussian and non-Gaussian flux distributions, variable PSD slopes, and evolving spectral curvature supports scenarios involving energy-dependent particle acceleration, turbulence, and/or magnetic reconnection within the jet.
