Exploring longterm spectral variations in blazars a detailed study of 3c 279 and 4c 31 03

Abstract

Blazars are among the most luminous extragalactic sources in the universe. Numerous studies have explored the non- thermal continuum emission from blazars, employing various emission models to elucidate the potential mechanisms at play. Most of these investigations focus on the strong flares exhibited by blazars. However, blazars are highly variable sources, with light curves that feature both major and minor flares, the latter being more frequent. Therefore, conducting long-term studies based on spectral and timing analyses is as crucial as those focusing on individual flare events. This thesis presents studies on two Fermi bright blazars 3C 279 and 4C 31.03 based on the systematic multi-wavelength data analysis. The work explores the spectral variations in different energy bands exhibited by these sources on a long-term basis. In the first work, we performed a detailed correlation/regression study of various spectral parameters obtained from the spectral analysis of long-term multi-wavelength data. For this purpose, we selected 3C 279 as a case study and utilized simultaneous Swift-XRT/UVOT and Fermi -LAT observations spanning from August 2008 to June 2022. The optical/UV, X-ray, and γ-ray spectra were individually fitted by a power-law to obtain the fluxes and indices in these energy bands. A combined spectral fit of simultaneous optical/UV and X-ray spectra was also performed to obtain the transition energy where the dominant emission mechanism changes from synchrotron to inverse Compton. The primary conclu- sion of this study is that long-term spectral variations at low energies are mainly caused by changes in the spectral index, while variations at high energies are attributed to changes in the break energy of the underlying particle spectrum. Systematic broadband SED modeling confirms that the EC peak energy shift can be caused either by variations in the break energy or by changes in the target photon field. The second study associated with this thesis explores the first major γ- ray outburst of the blazar 4C 31.03 observed by Fermi-LAT in January 2023. 4C 31.03 remains as sparsely studied source with less availability of observations in other wavelengths. This is the first time the source is investigated for its γ-ray characteristics and other properties. The temporal variability of the source was investigated using 7, 1, and 0.5 day binned γ-ray lightcurves. The statistical analysis of lightcurves using bayesian blocks and HOP algorithm identified three prominent flaring episodes of the source. We also performed a detailed broad-band SED modeling for three epochs. This study concludes that the production of γ-rays in 4C 31.03 likely occurs beyond the broad-line region (BLR). The flux variations are attributed to the variations in the magnetic field, Doppler factor and target photon field energy density. This study also notes a significant spectral transition during the outburst, especially at X-ray energies. The thesis also covers an elaborated flux distribution study of 3C 279 and 4C 31.03. For 3C 279, this study spans γ-ray, X-ray, and optical/UV bands using observations collected over 14 years. For 4C 31.03, the analysis is limited to the γ-ray spectrum due to the scarcity of X-ray and optical/UV data. The log- normal flux distribution observed for 3C 279 can be attributed to the Gaussian variations in the spectral index, while this inference may not be applicable to 4C 31.03.