Accretion Disk-Corona connection in Active Galactic Nuclei

Abstract

Active Galactic Nuclei (AGN), amongst the most luminous sources in the Universe (L = 1042 − 1046 erg s −1 ), emit radiation over the entire electromagnetic band. Though AGN are known to be powered by the accretion of matter onto their central supermassive black holes (SMBHs), the detailed structure of the accretion flow onto the central engine is still unclear. The emission of high energy radiation (X-rays and γ-rays) from the central part of the galaxy is one of the most distinguishable features of AGN from the normal galaxy. It is commonly believed that in the radio-quiet class of AGN, a compact region composed of energetic plasma situated in the vicinity of the central engine is responsible for the emission of the observed X-ray continuum and is known as the corona. Despite a growing number of studies of AGN in X-ray waveband, the physical nature, geometry and position of the corona are still unknown quantities. In my thesis, the work is directed to investigate the physical properties of the corona through broad band X-ray spectral analysis of the radio-quiet category of AGN. The systemic and detailed study was carried out using the hard X-ray data from the Nuclear Spectroscopic Telescope Array (NuSTAR) in the 3−79 keV energy band. Owing to its high sensitivity at hard X-ray energies (>10 keV) NuSTAR enabled the examination of the key features of the X-ray spectrum and to put constraints on the measurement of several physical properties of the corona (such as coronal temperature, optical depth etc.), which is one of the most important goals of this thesis. The first study presented in this thesis concentrates on constraining the nature of the X-ray corona in Seyfert 1 galaxies. With this aim, a sample of 130 type 1 sources with a count rate greater than 0.1 was collected from NuSTAR Master Catalogue between August 2013 and May 2022. A uniform analysis of the 130 sources was carried out using the phenomenological model to estimate the high energy turnover observed in the hard X-ray NuSTAR spectra of the sources. To constrain the physical parameters of the corona, further analysis was carried out using different physically motivated models in 48 sources, for which the high energy cut-off could be constrained. To understand the nature of the corona, possible correlations between various properties of the corona obtained from the physical model fits to the observed spectra and between various coronal parameters and physical properties of the sources, such as Eddington ratio and black hole mass etc., were investigated. In addition to finding the temperature of the corona in about four dozen Seyfert 1 galaxies via a homogeneous analysis, this thesis also investigated the coronal temperature variation and the possible reasons behind such variation observed in Seyfert 1 sources. From the NuSTAR Master catalogue, 21 sources with good signal-to-noise ratio (count rate > 0.1) data with multiple epochs of observations were selected to carry out a systematic spectral analysis of the X-ray spectra in the 3−79 keV band. From fitting physical models to the data for 72 epochs of 21 sources, the coronal temperature could be constrained in most of the observations, a few of which are found for the first time. Among the 21 sources, the variation in the temperature of the corona was ascertained at a 90% confidence level in two sources, namely, NGC 3227 and MCG+08-11-011. In MCG+08-11- 011, analysing the epochs in which we could constrain the temperature of the source, a “hotter-when-brighter” trend was noticed. The spectra of the source followed a “hotter-when-softer” nature with the reflection fraction, and the optical depth of the corona diminishing with the temperature of the corona. All these observations point to changes in the coronal geometry as a reason behind the variation in the coronal temperature of MCG+08-11-011. In NGC 3227, we found no notable correlation of kTe, Γ or R with flux, while a significant negative relation is found between τ and kTe. This could be due to more than one physical process at work in the source responsible for the observed change in kTe. In addition to the investigation of the coronal properties in Seyfert 1 type AGN, the thesis also investigated the timing and spectral properties of the Compton Thick Seyfert 2 AGN NGC 1068 observed using NuSTAR and XMM-Newton. For the first time, the coronal temperature was calculated for the source. The variation of the corona temperature was also checked between the epochs. The data analysed in this work comprised of (a) eight epochs of observations with NuSTAR carried out during the period December 2012 to November 2017, and (b) six epochs of observations with XMM-Newton carried out during July 2000 to February 2015. From timing analysis of the NuSTAR observations, the source did not show any variations in the soft band. However, on examination of the flux at energies beyond 20 keV, during August 2014 and August 2017, the source was brighter by about 20% and 30%, respectively, compared to the mean flux of the three 2012 NuSTAR observations as in agreement with the results found in the literature. From an analysis of the XMM-Newton data, no variation in the hard band (2−4 keV) was found between epochs and within epochs. In the soft band (0.2−2 keV), while the source was found to be not variable within epochs, it was found to be brighter in the epoch B (observation ID: 60002030004) relative to the epoch A (observation ID: 60002030002). By fitting physical models, the temperature of the corona was found to range between 8.46+0.39 −0.66 keV and 9.13+0.63 −0.98 keV with no detection of variation in the temperature of the corona in NGC 1068. From physical model fits to the observed spectra for several AGN, the thesis was able to find the temperature of the corona in many AGN, as well as the variation in the temperature of the corona in two sources, namely, NGC 3227 and MCG+08- 11-011. However, from spectral analysis alone, it is not possible to distinguish between different coronal geometries. X-ray polarimetric observations could yield the needed measurements to constrain the geometry of the X-ray-emitting corona in AGN. As of the writing of the thesis, the Imaging X-ray Polarimetry Explorer IXPE, launched on December 9, 2021, and sensitive in the 2−8 keV band, has observed four radio-quiet AGN. The final part of the thesis reports the first time measurement of X-ray polarization in the radio-quiet Seyfert 1 galaxy IC 4329A. The first-time measurement of the X-ray polarization in IC 4329A argues against a spherical lamp post geometry, however, favouring a conical-shaped corona in IC 4329A. The thesis finally ends with possible future work that needs to be undertaken to understand the central regions of AGN.