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.
Description:
The thesis is submitted to Deen Dayal Upadhyaya Gorakhpur University, Gorakhpur, under the supervision of Dr. Hum Chand & Prof. Ravi Shankar Singh.