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Multiwavelength Long Term Monitoring and Spectral Engery Distribution Modeling of Bright Active Galactic Nuclei Markarian 421

Biswajit Banerjee


Homi Bhabha National Institute


The studies reported in this thesis can be divided into three parts, namely, (a) the study of the performance of MAGIC telescopes below 100 GeV with the help of a New (image) Cleaning algorithm, (b) multiwavelength (MWL) study of the blazar Markarian 421 (Mrk 421), and (c) spectral modeling of Mrk 421 during a flare in 2010. The study on the performance of MAGIC telescopes below 100 GeV is important for the soft spectrum sources such as GRB, high redshift AGNs and pulsars. The sensitivity of the telescope at lower energies (a few tens of GeV) is poor primarily because of two reasons: poor gamma-hadron separation, and due to the presence of the noise produced by the Domino-Ring-Sampler (DRS4) chips. This is why it is difficult to identify the low energy events while lowering the analysis threshold. The New Cleaning algorithm showed that an improvement in the sensitivity of the MAGIC telescopes down to 40-50 GeV is possible with the standard triggered data. The work related to the analysis of very high energy gamma-ray data of Mrk 421 with MAGIC telescopes along with MWL data is aimed for the understanding of the source behavior in different wavebands. In this campaign, the VHE and X-ray observations were planned within 2 hours. During this campaign, the source was mostly found in low flux states. For a duration of 2 months in 2016, the source was found in a very low flux state in VHE gamma-rays and X-rays. The variability in the source lightcurve has been studied extensively with the help of hardness ratio and fractional variability. The long-term correlation between several lightcurves at different energy bands observed during this period has also been extensively studied (using discrete correlation function; DCF). In this study, a new spectral component at hard X-rays (15-50 keV) observed with Swift-BAT has been detected during the low state of the source in 2016 which does not agree with the log-parabola fit of the synchrotron hump. This thesis reports one of the very first studies of the flux distribution in an unbinned manner with two methods: the ’flux-profile’ method (similar to Kernel Density Estimator) and unbinned likelihood method, where the flux-uncertainties are also taken into account. These studies show that the MWL flux distributions mostly follow a LogNormal flux distribution which hints to a multiplicative process responsible for the variability of the source. The study related to the time-dependent leptonic modeling aims towards the characterization of a flaring episode of Mrk 421 in February 2010. The modeling of the spectral energy distribution (SED) of Mrk 421 is carried out with a time-dependent multi-zone radiation feedback model. The source showed a very high flux state in X-ray and VHE gamma-rays. In addition, a significant spectral slope hardening over a period of 24 hours can also be seen. On 15th February the SED was similar to the steady-state observed during 2009 (Abdo et al. 2011a). In the later days, the flux state changed significantly. On 17th February, the source was detected in a very high (and hard) state. With the help of the time- dependent leptonic model, all the three states during the outburst has been explained. The light-curve realizations for different energies for these days highlight the main differences between the role of different model parameters in shaping the emission profiles at different energies. This is one of the very first attempts made to understand a complete flaring episode using a cylindrical shaped emission region in a multi-zone scenario. In this study a general trend can be established which shows that as the flare evolves from a low- to a high-flux state, higher bulk kinetic energy is injected into the system with a harder particle population and a lower magnetic field strength.

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