Extragalactic Astrophysics Using 4m International Liquid Mirror Telescope

Abstract

A new facility, the 4m International Liquid Mirror Telescope (ILMT) is currently being commissioned at the Devasthal Observatory, Nainital. ILMT will perform a deep survey of all the celestial objects falling in its field of view, and is highly suitable to characterize their photometric variability. The uniqueness of the ILMT to observe the same field with a 1-day cadence makes it generally a useful facility for time-domain study of different types of galactic and extragalactic objects, with our main focus here on the photometric variability studies of active galactic nuclei (AGNs). AGNs are one of the most luminous persistent sources of electromagnetic (EM) radiation, with their luminosities much higher than that of a normal galaxy. One of the defining characteristics of AGNs is their rapid variability from minutes to year-long timescales in the entire EM spectrum. In the last four decades, AGN variability has been proven to be one of the most powerful tools to probe the structure and physics near the central parsec region around supermassive black holes. Albeit, our understanding of these sources remains contentious due to the statistically limited or biased samples. Nowadays, with the advent of various new time domain surveys such as the existing Zwicky Transient Facility (ZTF), the upcoming Large Synoptic Survey Telescope (LSST) and the ILMT, it has become possible to study the variability properties of large and homogeneous samples of AGNs without any observation bias. We have performed an extensive study of large homogeneous samples of AGNs, specially in blazars, on intra-night to year-long timescales to study the structure and dynamics of their central region. In this context, we have first worked on the development of a data reduction pipeline for the ILMT, that performs pre-processing and astrometry and aperture photometry of the data obtained in the TDI mode, with a long-term goal of characterizing AGNs in the ILMT field with a 1-day cadence. Testing the pipeline on the data obtained with ILMT in the commissioning phase, we have achieved sub-arcsec accuracy with a standard deviation ∼0.24 arcsec in astrometry and ∼0.16 mag in the photometric calibration. Next, we have studied the intranight optical variability (INOV) of 53 blazars based on their ZTF light curves along with a 20-fold control sample of radio-quiet quasars (RQQs). These two classes of powerful AGN represent opposite extremes of jet activity. Our analysis of their ZTF light curves has revealed some strong INOV events which, although not exceptionally rare for blazars, are indeed so for RQQs. This shows that a blazar-like INOV level can also be attained by RQQs, albeit very rarely, and hints the presence of micro-jets of optical emission performing bulk relativistic motion in RQQs too. We also extend our INOV study to a rare subclass of blazars emitting in TeV (TeV blazars), based on observations from 1.3m Devasthal Fast Optical Telescope (DFOT). Here we have tried to understand the role of dominant superluminal radio knots in the parsec scale jets in the INOV of AGNs. Studying a well selected sample of 6 TeV blazars, with fairly high degree of optical polarisation, but lacking superluminal radio-knots, we found a zero INOV DC for these TeV blazars demonstrating that the presence of dominant superluminal radio knot(s) in the parsec-scale jet constitutes a key diagnostic for INOV detection and while a high degree of optical polarisation is also an important marker, it alone is not a sufficient diagnostic for INOV detection. Following the intranight flux variations in the blazars, we have also searched for any possible universality in the long-term color behavior of a large sample consisting of 897 blazars comprising 455 BL Lacs and 442 Flat Spectrum Radio Quasars (FSRQs) based on ZTF monitoring. We found the color behavior of BL Lacs to be dominated by a ‘bluer when brighter’ (BWB) trend, whereas a ‘redder when brighter’ (RWB) trend was found in most FSRQs. The BWB trend in BL Lacs can be explained with a shock-in-jet model where electrons at the front of a shock are accelerated to very high energies and cool by radiative cooling, making the high energy bands more variable. The RWB trend in FSRQs arises due to the addition of a redder jet flux to a constant bluer flux from the accretion disk. Further, we have searched for the periodicities based on the ZTF light curve in a sample of 2103 blazars over a duration of ∼3.8 years. We have found quasi-periodic oscillations (QPOs) in 5 blazars which were independently confirmed in r and g bands using three methods: Lomb Scargle Periodogram (LSP), Weighted wavelet-Z (WWZ) transform and Phase dispersion minimization (PDM) technique. These QPOs are most likely to originate from the variation in the observed Doppler factor in a precessing jet of high Lorentz factors, aligned closely to the observer or the movement of a plasma blob along an internal helical structure. In summary, in this thesis, for our broader motivation of AGN monitoring program with 4m ILMT, we have developed a robust data reduction pipeline for the TDI mode observations with the ILMT. Next, by using large samples of blazars and RQQs, we have constrained the INOV behaviour these AGNs as a population. Further, using the INOV observations with the DFOT, we have demonstrated the presence of superluminal radio knot as a key marker for INOV. Using the ∼3.8 year long light curves of blazars from ZTF, we have also investigated the origin of colour behaviour in blazar population and have searched for any possible universality in their colour behaviour. The long term light curves have also enabled us to search for any possible periodic signatures in the light curves of large sample of blazars and their origin. These observational constraints have allowed us to refute or confirm among many theoretical models for the variability of the central engine of the AGNs.