Investigating the nature and structure of the inner regions in Active Galactic Nuclei
Date & Time :
Even with large telescope apertures and vast improvements in detector technology, resolving the inner regions of active galaxies remains a formidable challenge. Nevertheless, understanding the innermost regions of these galaxies is critical to understanding the galaxy evolution and the structure and dynamics of matter in the vicinity of a Supermassive Black Hole (SMBH). In this thesis, I have explored indirect methods to further our understanding of the innermost regions in Active galactic nuclei (AGN). As a first step, we use the reverberation mapping technique to estimate accretion disk sizes for a sample of AGN. We look for a correlation between the estimated disk size and the known physical parameters, i.e., SMBH mass and luminosities. Our results show that the accretion disk sizes computed using this method are, on average, 3.9 times larger than the predictions of the Shakura Sunyev standard disk model. Additionally, we find a weak correlation between the obtained accretion disk sizes and the SMBH mass and AGN luminosity. Further, we calibrate the narrow-band photometric reverberation mapping (PRM) technique to develop tools for a large systematic narrow band PRM project with regard to the upcoming Vera Rubin Observatory- Legacy Survey of Space and Time (VRO-LSST). This can enable the measurement of SMBH mass and BLR sizes for a large number of quasars at a fraction of the telescope's time. To develop an optimum strategy for executing a successful PRM project, we perform extensive simulations to test the effect of cadence, variability of the light curves, and the duration of cadence. In the next chapter, we use the method of microvariability to study the dichotomy between AGNs with and without detected jets. Through a monitoring program spanning 53 sessions of a minimum duration of 3 hours, we demonstrate that the jets induce significant microvariability in the optical bands in these AGNs, and the AGNs with confirmed jets are about 3 times more variable on short time scales than the AGNs without a confirmed jet. Moreover, the presence of relativistically beamed jets induces significant microvariabilities approaching blazars in these sources. Finally, we perform statistical analysis on a large sample of low luminosity AGNs to investigate the dynamics of matter in BLR and infer the properties of a particular class of AGN, the Narrow Line Seyfert galaxies (NLSy1 galaxies). Based on a sample of 144 NLSy1 and 117 BLSy1 galaxies, we find that the NLSy1 galaxies are more likely to have outflow signatures than their broad-line counterparts hinting towards the disk wind origin of the material in BLR. Through a principal component analysis on this data, we find out the principal components for NLSy1 galaxies differ from the BLSy1 galaxies, which points that the NLSy1 galaxies are not just a subclass of BLSy1 galaxies. During this presentation, I will discuss these important findings generated during the course of this thesis and their significance in understanding the innermost regions of AGN.
About Speaker :
Vivek Jha is a Ph.D. student registered at DDU Gorakhpur University. This is his pre-Ph.D. thesis submission seminar.