Broadband spectral analysis and variability of AGNs

Abstract

Active Galactic Nuclei (AGNs) are among the most powerful persistent sources of electromagnetic energy in the Universe, powered by accretion of mass onto supermassive black holes. Studying their broadband spectra and variability pat- terns can provide information on the geometry, physical conditions and energetics of the accretion disk-corona system. This thesis investigates the physical pro- cesses governing AGN mechanism through a combination of broadband spectral modelling and variability informed X-ray analysis using data from AstroSat, NuS- TAR and XMM-Newton. Long term spectral analysis of the Seyfert 1 galaxy, Mrk 279, using multi-epoch data was conducted to examine the evolution of the source’s flux state and the associated variations in individual components. The study revealed significant source variability without consistent correlation between photon index and continuum intensity. Variations in spectral shape fol- lows only the long term flux state variability and not short timescale variabilities. Flux variability without accompanying spectral shape variability can undermine small changes in physical parameters, necessitating variability analyses along with broadband spectroscopy. Incorporating temporal information with spec- tral modelling, a novel method for modelling the energy dependent variability is introduced. The method is demonstrated by applying to an XMM-Newton observation of the NLS1 galaxy, Mrk 335, whose spectrum can be described by (i) a partial covering absorber modifying the primary emission, (ii) a warm and hot coronal model and (iii) gravitationally blurred reflection along with primary emission. The source’s intrinsic variability, quantified through fractional rms variability amplitude can be utilised to break the degeneracy between the spec- tral models. The study presents a technique to estimate fractional rms from the spectral model and identify the model parameters that best recreates the observed variability pattern. Together the studies consider the different com- plementary aspects of AGN behaviour, demonstrating that a multi-domain ap- proach combining long term spectral evolution along with variability-informed modelling provide a powerful tool to understand AGN physics.