TRANSITIONS AND PERSISTENCE OF BLAZAR STATE IN BEAMED RADIO QUASARS

Abstract

A compact region at the centre of a galaxy known as an active galactic nucleus (AGN) generates a significant amount of energy that is not directly due to stars. It is believed that the accretion of materials onto supermassive black holes (SMBH) powers AGNs. Blazars are radio-loud AGNs whose observed radiation is predominantly non-thermal and arises from a relativistic jet that is roughly towards the observer having angle < (5-10)◦ to the line of sight. Hence its radiation appears highly Doppler-boosted and completely dominates the radiation coming from the host galaxy and accretion disk. Blazars are well known for having high and variable fractional optical linear polarization (popt), and their jets frequently exhibit superluminal motion and flux variability from radio to γ rays on time scales as short as minutes. Blazars consist of BL Lacs objects and flat spectrum radio quasars (FSRQs), the former having featureless optical spectra and the latter having strong emission lines. The blazar subset among the FSRQs having popt > 3% is known as the High polarization Quasars (HPQ) class. Several studies have previously looked at the issue of the duty cycle of high optical polarization state (i.e., blazar state) among the FSRQs and it has been estimated that about two-thirds members of a FSRQ exhibit a blazar state at a given time. This raises the question of how long an individual FSRQ, upon entering the blazar state, remains in that state. This question forms a key element in the physics of quasars which we have studied in the first part of this thesis. The work in this thesis takes an important step by addressing this fundamental question directly for individual quasars. We did this by using large, unbiased samples of the core dominated (i.e., flat-spectrum) radio quasars whose optical polarization state had been established during the 1980s. The latest available, high-quality optical light curves from the Zwicky Transient Facility (ZTF) survey were used to track the blazar status of its members individually. The polarization data from the Opto-polarimetric survey RoboPol were also used to track the blazar status. This strategy allowed us to cover an exceptionally long time baseline of four decades and led us to find out that ∼ 90% of radio quasars persist in their blazar mode (i.e., HPQ state) for 3-4 decades, though state transitions on year-like time scales may also occur in some cases, which is most likely associated with short-term processes such as the formation and ejection of relativistic plasma blobs (VLBI knots) from the active nucleus. Secondly, intranight optical variability (INOV) studies of AGNs have played an im portant role in unravelling the nature of their central engines. Hence, we also made a first attempt to characterise the INOV of blazars with extreme optical luminosity (1046 −1048 erg s −1 ) located at high-z (>1.5). Due to the high-z of the selected blazars, the monitored optical radiation fall in their rest-frame UV emission. This effect allowed us to character ise the variability of the UV emission emerging from blazars on hour-like or shorter time scales for the first time. For this, we carried out intranight optical monitoring of 14 flat spectrum radio quasars (FSRQs) located at high redshifts (1.5 < z < 3.7), in 42 sessions of median duration ∼ 5.4 hr. These sources were grouped into two samples distinguished by low and high fractional polarization measured in the optical, with the division taken at popt = 3% which resulted in (i) nine low-polarization sources with popt < 3% and (ii) five high-polarization sources with popt > 3%. The photometric analysis of 14 FSRQs (nine LPFSRQs and five HPFSRQs) located at high redshifts revealed that the intranight variab ility of rest-frame UV emission has possibly opposite polarization dependences for the intranight variability of the UV and optical radiations from blazars. This posits that the non-thermal UV emission of blazars is caused by a relativistic particle population differ ent from that radiating up to near-infrared/optical frequencies. Furthermore, we extended our INOV study by using a unique sample of low-mass AGNs (LMAGNs, ∼ 106 M⊙) at low-z (. 0.1) that are 2-3 orders of magnitude less massive than SMBH (SMBH ∼ 108 − 109M⊙). So, we made a first attempt to charac terise the INOV properties of such LMAGNs. We mainly investigated here whether the blazar-level activity can even be sustained by less massive central engines of galaxies, us ing a well-defined, representative sample of 12 LMAGNs already detected in X-ray and radio bands. We found that the INOV properties of these LMAGNs are similar to that of blazars which hint at the presence of relativistic jets in them. In summary, based on detailed analysis, our study suggests that (i) ∼ 90% of beamed radio quasars persist in their blazar mode (i.e., HPQ state) for 3-4 decades. The long-term stability of the blazar mode (i.e., HPQ state) and its transience on a year-like time scale can occur in tandem in beamed radio quasars; (ii) our photometric analysis of 14 FSRQs (nine LPFSRQs and five HPFSRQs) located at high redshifts suggests that the non-thermal UV emission of blazars arises from a relativistic particle population different from that radiating up to near-infrared/optical frequencies; and (iii) our INOV study of LMAGNs suggests that a blazar-like INOV activity exists in LMAGNs which hints at the presence of relativistic non-thermal jets in LMAGNs. In the first chapter, we introduce AGNs, different classes of AGNs and their unification scheme. The second chapter deals with telescopes, instruments, observations and data reduction. The next three chapters focus on the investigation of the above-mentioned fundamental question, i.e., transitions and persistence of blazar state in beamed radio quasars and the INOV characterisation of AGNs with extreme optical luminosity and low black hole mass. In the sixth chapter, we finally conclude the thesis.