The optical-GeV connection in fermi blazars

Abstract

Blazars are a class of active galactic nuclei (AGN) that have relativistic jets that are oriented close to the observers line of sight. They display ux variability across the en- tire accessible electromagnetic spectrum from low energy radio to high energy -rays. They are classi ed into BL Lacertae objects (BL Lacs) and at spectrum radio quasars (FSRQs) based mainly on the absence or presence of broad emission lines in their op- tical/IR spectra. They dominate the extragalactic -ray sky and are also suggested to be the possible sources of astrophysical neutrinos. The recent detection of neutrinos by the IceCube collaboration has been found to be closely associated with aring blazars. In the leptonic model of emission from blazar jets, the low energy emission is from acceleration and cooling of relativistic electrons through synchrotron emission process and the high energy emission is through inverse Compton scattering o jet relativistic electrons that produce the synchrotron emission. The seed photons for the inverse Compton process come either from inside the jet (synchrotron self-Compton or SSC), or from outside the jet (external Compton or EC). Whereas in the hadronic model of emission from blazar jets, relativistic protons also contribute to the high energy emission through the proton synchrotron emission or photo-pion production processes. An e cient way to constrain the leptonic v/s hadronic emission from blazar jets is through modelling of the broad band spectral energy distribution of blazars. However, this is hindered by (i) the di culty in accumulating simultaneous or near simultane- ous data over a range of wavelengths and (ii) complexity of the available models in explaining the observed SED. An alternative and relatively cheap method to constrain the leptonic v/s hadronic emission in blazars is via the analysis of ux variations in the optical and -ray bands. In the leptonic model of emission from blazar jets, as the relativistic electrons in the jet are responsible for the optical and -ray emission a close correlation between the optical and -ray ux variations are expected. Alternatively in the hadronic model of emission from blazar jets, as the optical emission is from electron synchrotron and the -ray emission might be due to proton synchrotron, a close correlation between optical and -ray ux variations are not expected. In this thesis, I mainly concentrated on the question "Is leptonic model able to explain the optical and -ray ux variations and optical polarization behaviour in blazars?" I have followed the following approaches to address the above question (i) to con- strain the leptonic scenario in blazars through correlation analysis of ux variations in the optical and -ray bands (ii) characterize the -ray variability characteristics of di erent categories of blazars on month like time scales and (iii) characterize the correlation between ux and polarization variations in blazars to constrain the connection between di erent emission regions in the jets of blazars. To achieve the rst objective, I analyzed ten years (2008 - 2018) of multiband data on a sample of ve FSRQs (3C 454.3, PKS 1510-089, 3C 279, 3C 273 and CTA 102) and three BL Lac objects (AO 0235+164, OJ 287 and PKS 2155-304). In the case of FSRQs, I noticed (i) correlated optical and -ray ux variations, (ii) optical are without a -ray counterpart and (iii) -ray are without an optical counterpart. In all the three BL Lacs analyzed in this thesis, I found correlated optical and -ray are. Our SED modelling of those epochs indicates that correlated optical and -ray ux variations are mostly driven by changes in the bulk Lorentz factor, while -ray ares without optical counterparts are due to an increase in the bulk Lorentz factor and/or increase in the electron energy density and optical

ares without -ray counterparts are due to an increase in the magnetic eld. Details are given in Chapter 3 and Chapter 4 of the thesis. The second objective of characterizing the long term -ray ux variability characteristics of blazars is addressed by analyzing the one month binned light curves of 1120 blazars, of which 481 are FSRQs and 639 are BL Lacs. Monthly binned light curves on these sources were generated for a period of about 9 years from 2008 August to 2017 December and the variability was quanti ed by excess variance (Fvar). On month like time scales, 371/481 FSRQs are variable (80%) while only about 50% (304/639) of BL Lacs are variable. FSRQs are thus found to be more variable than BL Lac objects. Large Fvar in FSRQs is also con rmed from the analysis of ensemble structure function. Details of this are given in Chapter 5 of the thesis. The third objective of the thesis is addressed in Chapter 6 of the thesis where I made an attempt to systematically study the optical ux and polarization variations in the at spectrum radio quasar 3C 279. The total ux and polarization data in the optical V-band cover a period of about 10 years from 2008􀀀2018 August. During this period the source varied in optical brightness by about 5 magnitudes. On day like timescales, I identi ed eleven epochs wherein statistically signi cant correlation between total and polarized ux exists. Of these, on ve epochs total and polarized ux are correlated while on the remaining six epochs total and polarized

ux are anti-correlated. The varied levels of correlation observed between optical ux and polarization degree on day like timescale point to the presence of multiple emission regions in the jet of 3C 279.