General physical characteristics of γ-ray emitting beamed AGNs in fermi era

Abstract

The research work presented in this thesis is aimed to provide a detailed understanding of the radiative processes powering the jets of the blazars, a class of AGNs with jet pointed towards the line of sight to the observer. Another important aim of this work is to study the multi-frequency properties of a new class of γ-ray emitting narrow-line Seyfert 1 (γ-NLSy1) galaxies using various observational tools and to compare them with that known from powerful blazars. The goal of understanding the physical characteristics of blazars is achieved by studying the broadband flux variability and spectral energy distribution (SED) modeling of a carefully selected sample of blazars. The aim of the broadband variability study is to provide a general physical scenario, which allows one to put the observed variation from blazars across several decades of frequencies in a coherent context. Extremely fast hr scale flux variations are observed from all the sources studied here, including nearby BL Lac object Mrk 421 (z = 0.03) which showed minute scale hard X-ray (3−79 keV) flux variability during its 2013 April X-ray outburst. Moreover, by adopting a SED odeling approach, attempts are made to understand the causes of high amplitude γ-ray flux variations observed from these objects. To do this, a simple one zone leptonic emission model was also developed during the course of the thesis. The main reason of the 2014 April and 2015 June γ-ray outbursts of 3C 279 (z = 0.536) and 2011 December γ-ray flare of distant blazar S5 0836+71 (z = 2.17) is found to be due to sudden acceleration of the jet. On the other hand, minute scale variability seen from Mrk 421 is explained on the basis of magnetic energy dissipation and reconnection events. The multi-wavelength observations of 3C 279 revealed that a single zone leptonic emission model successfully reproduces the γ-ray flares of 2014 April and 2015 June, however, it fails to explain the uncorrelated flux variations and a hard γ-ray pectrum seen during 2013 December event. A two zone leptonic emission model is used to match the observations. All these observations hint for the presence of aiv variety of the radiative processes working in the 3C 279 jet (and possibly in other sources as well) and their dominance over each other, as seen during different high activity periods. The launch of the Fermi Gamma-ray space telescope in the year 2008 led to the first detection of γ-ray emission with high confidence from about half-a-dozen radioloud NLSy1 galaxies. This discovery clearly indicates the presence of relativistic jets in these sources similar to that of blazars. With the motivation to understand the nature of γ-NLSy1 galaxies vis-a-vis blazars, few diagnostic tests are carried out, namely, intranight optical variability (INOV), γ-ray spectral properties, and broadband SED modeling. It is found that: (1) these sources show large amplitude (>3%) INOV with a duty cycle of about 80%, (2) their γ-ray spectra exhibit a significant curvature, and (3) their broadband SEDs have the typical double hump structure and the high energy hump can be explained due to external Compton process. Thus, based on the observations covering a wide range of the electromagnetic pectrum, it can be concluded that γ-NLSy1 galaxies have all the properties similar to blazars and could well be the low black hole mass counterparts of flat spectrum radio quasars.