Radiation mechanisms of VHE gamma-ray sources a thesis submitted for the award of doctor of philosophy in department of physics, Pondicherry university, Pondicherry Amit Shukla [Ph.D Thesis]

Thesis Supervisor Prof. Anupama, G. C.

Doctor of Philosophy Pondicherry University, Puducherry 2013

Active galactic nuclei (AGN) are one of the most luminous objects in the uni-verse. They are the sub-class of galaxies which emit extremely luminous emissionfrom the nuclear regions of the galaxy. This emission is spread widely across the electromagnetic spectrum from radio wavelengths to -rays. This radiation from AGN is believed to be the result of accretion of matter onto the supermassive black hole of mass 108- 1010 M⊙ at the center of the host galaxy. The AGN are classified by their random pointing directions to the observer in the unified scheme. AGN are mainly divided into three classes : (i) Seyfert galaxies, (ii) quasars and (iii) blazars.
(i) Seyfert galaxies have modest luminosities and they are best studied since they generally lie near to us; (ii) quasars are more luminous than the host galaxy and found further away, and (iii) blazars are characterized by nonthermal emission ex-
tending from radio to high energies. The broadband radiation originates within a relativistic jet that is oriented very close to the line of sight of the observer. The extragalactic TeV astronomy began with the detection of the nearby (z=0.031) blazar Mrk 421 above 500 GeV by the Whipple observatory [1]. Spectral energy distribution (SED) of the high energy peaked TeV blazars show two broad
peaks. The first peak is located between infrared to X-ray energies and the second peak at -ray energies. It is believed that the first peak of the SED originates due to synchrotron radiation by relativistic electrons gyrating in the magnetic field of the jet. The origin of the high energy GeV/TeV peak is still under debate. This high energy peak might originate either due to interaction of electrons with photon field via Inverse Compton (IC) scattering as in leptonic models [2, 3, 4] or due to interaction of protons with matter, magnetic field [5, 6] or photon fields as in the hadronic models. The existing data from multiwavelength observations are not good enough to
constrain the dominant emission mechanisms in the jet that are responsible for the high energy bump. The main reason behind this is the lack of the data in the energies from 100 MeV to hundreds of GeVs. In the past few years, ground based Atmospheric Cherenkov Telescope (>100 GeV) and the space based Fermi-LAT telescope (30 MeV- 300 GeV) have started providing data in this energy range. The multiwavelength observations from radio to very high energy (VHE) -rays have started to provide better constraints on the AGN models [7] and also enhance our understanding about the these sources. A multiwavelength study of Mrk 421 and Mrk 501 are presented in this thesis. The very high energy -ray data obtaining using HAGAR array are combined with
archival data from Fermi-LAT, RXTE-ASM, Swift-BAT, Swift-XRT, RXTE-PCA, SPOL and OVRO for a multiwavelength study. The observed multiwavelength SED is explained by using a one zone homogeneous SSC model [8]. We have attempted
to obtain SEDs for different flux states using multiwavelength data of Mrk 421 and Mrk 501. The evolution of SED during a giant flare in February 2010 from Mrk 421 is also studied in detail. This study of TeV blazars shows that the observed broadband
SED is oriented by Synchrotron self Compton (SSC) mechanism. It also suggests that the electron population in the jet is accelerated by shocks which are present in the jets. Modeling of the flux variability from Mrk 501 is also attempted to constrain the
physical properties and emission mechanisms. Recently observed minute timescale variability of Mrk 501 at TeV energies has imposed severe constraints on jet models and TeV emission mechanisms. We present a viable model to explain this fast
variability. Details of HAGAR telescope array located at an altitude of 4300 m in IAO, Hanle are also presented in this thesis, with a detailed description of the instrument. In addition to the telescope system, data analysis techniques adopted to detect point
sources, and the basics of atmospheric Cherenkov technique are described.