Active Galactic Nuclei (AGN), refers to the existence of energetic phenomena in the nuclei, or central regions, of galaxies which can not be attributed to stars. AGN has a super-massive black hole at its center which accretes matter. Two major classes of AGNs are radio-quiet and radio-loud AGN. In my Ph.D. thesis, I have worked on a sub-class of radio-loud AGN called blazars. The blazars, I selected for my studies are TeV emitting blazars and known as TeV blazars. Blazars emit radiation in the complete EM spectrum. Their flux and polarization are highly variable on all diverse timescales and emission is predominately nonthermal.

My thesis results has 2 parts: (1) Multi-wavelength (optical/UV, and X-ray) observations of the TeV blazar PKS 2155 - 304 with XMM-Newton, (2) Multi-band optical observations of 3 TeV blazars; on diverse timescales.

(1) Using ~ 12 years XMM-Newton archival simultaneous data in optical/UV and X-ray bands of PKS 2155 - 304, we studied flux and spectral variability on diverse timescales and have tried to model the spectral energy distribution (SED). In a specific observation, we noticed the evidence of particle acceleration and synchrotron cooling both are at work for the first time. We also estimated the various parameters of SEDs.

(2) In multi-band optical studies of 3 TeV blazars, we have made new observations using 5 telescopes (2 in ARIES, 1 in Bulgaria, 1 in Greece, and 1 in Serbia). We have studied flux and color variations in these 3 TeV blazars on diverse timescales as short i.e. few tens of minutes and as long i.e. few years. We discuss the possible physical mechanisms most likely responsible for the observed flux variability.