Spectral characteristics of blazars: A detailed study of Mkn 421 and S5 0716+714

Abstract

The broadband SED of blazars is often modeled by a broken power-law function, in- dicating that the underlying electron distribution follows a broken power-law shape. However, one of the long-standing problems has been that the observed difference in the spectral index could not be explained solely by the radiative loss interpre- tation of the broken power-law electron distribution. Additionally, high-resolution observations have revealed curvature in the X-ray spectra of high-energy-peaked blazars. In the first part of the thesis, we aim to thoroughly investigate the radia- tive loss interpretation of the broken power-law electron distribution by studying the blazar spectrum around the synchrotron peak. We conduct a detailed analysis of the characteristic photon energy of blazar Mkn 421, where the spectral index changes significantly. Our findings indicate the presence of a strong anti-correlation between the spectral indices before and after this characteristic photon energy. This result contradicts the interpretation based on simple radiative loss for the characteristic photon energy, and alternative scenarios are thus discussed. Further, we carried out a detailed analysis on the observed curvature in the broadband X-ray spectrum of blazar Mkn 421. The curvature in the blazar spectrum can provide insights into the particle dynamics within jets. Our study suggests that the curvature in the X-ray spectra are consistent with a scenario where particle acceleration is governed by Bohm-type diffusion, and the spectra beyond the synchrotron peak are modulated by the radiative loss process. The latter part of this thesis focuses on a long-term, multi-wavelength spec- tral study of another blazar S5 0716+714. All available observations in optical/UV, X-ray, and gamma-ray wavebands of the source by Swift-UVOT/XRT or Fermi- LAT from April 2005 to January 2023 were used, and the spectra in each energy band were explored using power-law/log-parabola functions. A detailed correlation study between the best-fit parameters was performed, and our results suggest that the spectral changes observed during high flux states could be associated with the spectral energy distribution shifting towards the blue end. Furthermore, the broad- band SED during two distinct flux states can be successfully fitted by considering synchrotron, synchrotron self-Compton, and external Compton emission processes. The flux enhancement of the source is predominantly associated with an increase in the bulk Lorentz factor. Additionally, we find that the model curves corresponding to variations in the bulk Lorentz factor have the potential to explain the observed correlations between the spectral parameters. Our study thereby suggests that the spectral variations of blazar S5 0716+714 are primarily associated with changes in the bulk Lorentz factor of the jet.