AR-2019-2020

region of black hole X-ray binaries and active galac- tic nuclei has a broken power-law shape with a char- acteristic break timescale T B . If the disk and the jet are connected, the jet variability may also con- tain a characteristic timescale related to that of the disk-corona. Recent observations of the blazar Mrk 421 have confirmed the broken power-law shape of the PSD of its jet X-ray variability. We model the time variability of a blazar, in which emitting particles are assumed to be accelerated by suc- cessive shock waves flowing down the jet with a varying inter-shock timescale ( T IS ). We investi- gate the possible relation between the characteris- tic timescales in the disk and jet variability based on the above model, along with mathematically and physically simulated disk variability. We find that both the PSD of the jet and disk variability may have a broken power-law shape but the break timescales are not related in general except only in systems with a small range of BH mass. The break in the jet and disk PSD are connected to the interval between large amplitude outbursts in the jet ( T IS ) and to the viscous timescale in the disk, respectively. In frequency bands where mul- tiple emission processes are involved or emission is from lower energy particles, the break in the PSD may not be prominent enough for detection. This study has been done in collaboration with Sagnick Mukherjee, and Kaustav Mitra. Physical inference from the γ -ray, X-ray and op- tical time variability of a large sample of Fermi blazars We present cross-correlation studies of γ -ray (0.1- 300 GeV), X-ray (0.2-10 keV) and optical (R-band) variability of a sample of 16 blazars during 2008- 2016. The light curves are from the Fermi -LAT, Swift-XRT , and Yale-SMARTS blazar monitoring programme. We stack the discrete cross-correlation functions of the blazars such that the features that are consistently present in a large fraction of the sample become more prominent in the final result. We repeat the same analysis for two subgroups, namely, low synchrotron peaked (LSP) and high synchrotron peaked (HSP) blazars. We find that on average the variability at multiple bands is cor- related with a time lag consistent with zero in both subgroups. We describe this correlation with a lep- tonic model of non-thermal emission from blazar jets. By comparing the model results with that from the actual data, we find that the inter-band cross-correlations are consistent with an emission region of size ∼ 0 . 1 pc within the broad line region for LSP blazars. We rule out large changes of mag- netic field ( > 0 . 5 Gauss) across the emission region or small values of magnetic field ( ∼ 0 . 2 Gauss) for this population. We also find that the observed variability of the HSP blazars can be explained if the emission region is much larger than the distance to the broad line region from the central black hole. This study has been done in collaboration with An- wesh Majumdar, Kaustav Mitra, C. M. Urry, C. D. Bailyn, and Prantic Nandi. Suchetana Chatterjee, and Nishikanta Khandai Cosmological simulation of galaxy groups and clus- ters. I. Global effect of feedback from active galactic nuclei In this study, we quantify the properties of the gas and dark matter around active galactic nu- clei (AGN) in simulated galaxy groups and clus- ters and analyze the effect of AGN feedback on the surrounding intra-cluster (group) medium. Our re- sults suggest downsizing of AGN luminosity with host halo mass, supporting the results obtained from clustering studies of AGN. By examining the temperature and density distribution of the gas in the vicinity of AGN, we show that due to feedback from the central engine, the gas gets displaced from the centre of the group/cluster resulting in a reduc- tion of the density but an enhancement of temper- ature. We show that these effects are pronounced at both high and low redshifts and propose new ob- servables to study the effect of feedback in higher redshift galaxies. We also show that the average stellar mass is decreased in halos in the presence of AGN feedback confirming claims from previous studies. Our work for the first time uses a fully cosmological-hydrodynamic simulation to evaluate the global effects of AGN feedback on their host dark matter halos as well as galaxies at scales of galaxy groups and clusters. This work has been done in collaboration with Rudrani Kar Chowd- hury, Anto Lonappan, and Tiziana Di Matteo .