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The optical-GeV connection in fermi blazars a thesis submitted for the award of degree of doctor of philosophy in the department of physics, Pondicherry university, Puducherry

Bhoomika

The optical-GeV connection in fermi blazars a thesis submitted for the award of degree of doctor of philosophy in the department of physics, Pondicherry university, Puducherry [Thesis] Bhoomika - Bangalore Indian Institute of Astrophysics 2020 - xvii, 161p.

Ph.D. Thesis, Pondicherry University, Puducherry



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.


Blazars
Extragalactic
Flux variation
Gamma-ray emission

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