Broadband Spectral Study of Blazars

Abstract

Blazars belong to a subclass of radio-loud active galactic nuclei (AGN) with a relativistic jet directed towards the observer. The spectral energy distribution (SED) of a blazar extends from radio to -ray energies and is generally characterized by a double hump structure. The low energy hump peaking in the optical to X-ray energies and is explained by synchrotron process whereas the high energy hump with a peak located in γ-ray band and is mainly explained by inverse-Compton (IC) scattering of low-energy photons. The X-ray spectrum in most of the high enegy peaked blazars (HBLs) exhibits mild/strong curvature which can be well-fitted with a log-parabola function. However, for a better understanding of the intrinsic curvature and its evolution, a physically motivated synchrotron emission model is required. The main contribution of this thesis is to develop synchrotron and synchrotron self-Compton (SSC) emission models with different particle distribution of blazar-jets and their validation attributing to the unique curved spectral features observed in blazars. We have made multi-wavelength study of high-energy peaked blazar (HBL), Mkn 421 and extreme HBL (EHBL) source, 1ES 0229+200 using observations with various instruments including AstroSat−LAXPC, SXT, UVIT, Swift−UVOT, XRT, Fermi-LAT, and MAGIC. We present a time-resolved X-ray spectral study of the high peak blazar (HBL) source, Mkn 421, utilizing the simultaneous observations from LAXPC and SXT instruments onboard AstroSat during its flaring state. We fitted the observed X-ray spectrum with the synchrotron emission from various particle energy distributions, viz. max , EDD, EDA, and log parabola models. We found that although all these models fit the spectra, the EDD and EDA models were marginally better. The time-resolved spectral analysis allowed us to study the correlations between the spectral parameters of different models. In the simplest and direct approach, the observed correlations are not compatible with the predictions of the max model. While the EDD and EDA models do predict the correlations, the values of the inferred physical parameters are not compatible with the model assumptions. Thus, we show that the spectrally degenerate models can be distinguished based on their spectral parameter correlations (especially those between the model normalization and spectral shape) making time-resolved spectroscopy a powerful tool to probe the nature of such systems. Further, to test whether the correlation results from the long-term observations are consistent with the one obtained from short-term flare, we have performed a detailed analysis of the X-ray spectra of the blazar Mkn 421 using Swift-XRT observations and quantified the correlations between spectral parameters for different models. We show that the results from the long-term spectral-parameter correlations are consistent with those obtained from the single flare. The consistency of the results obtained from the long and short-term evolution of the source underlines the reliability of the technique to use the spectral-parameter correlations to distinguish various physical models of the blazar-jet emission. We have also studied broadband SED of an EHBL source, 1ES 0229+200, using quasi-simultaneous observations with various instruments including MAGIC, Fermi-LAT, AstroSatLAXPC, SXT, UVIT, and SwiftUVOT. We investigate the one-zone synchrotron and synchrotron self-Compton (SSC) model, employing diverse particle distributions such as the log parabola, broken power law, max , EDD, and EDA models to fit the broadband SED of the source. Our findings indicate that both peaks in the SED are well described by the one-zone SSC model across all particle distribution models. We estimate the jet power for different particle distributions. The estimated jet power for broken power law particle distributions is found to be on the order of 1047 (1044) erg s1 for a minimum electron energy min 10 (104). However, for intrinsically curved particle energy distributions (e.g., log parabola, EDD, and EDA models), the estimated jet power is 1044 erg s1. The SED fitting at five epochs enables us to explore the correlation between the derived spectral parameters of various particle distribution models. Notably, the observed correlations are inconsistent with the predictions in the max model, although the EDD and EDA models yield the correlations as expected. Moreover, the estimated physical parameter values are consistent with the model assumptions.