AR_final file_2018-19

the X-ray spectral shape, we perform a detailed study of the X-ray spectra of the blazar MKN 421, using an analytical model, where the elec- tron diffusion from the particle acceleration site is energy dependent. The resultant synchrotron spectrum is again determined by three parameters, namely, the energy index of the escape time-scale, the quantity connecting the electron energy to the observed photon energy, and the normalization. The X-ray observations of MKN 421, during 2012 July to 2013 April by Nuclear Spectroscopic Tele- scope Array and Swift-XRT are investigated using this model, and we find a significant correlation between model parameters and the observational quantities. Additionally, a strong anti-correlation is found between the fit parameters defining the spectral shape, which was not evident from ear- lier studies using empirical models. This indicates the flux variations in MKN 421 and possibly other blazars, may arise from a definite physical process that needs to be further investigated. This work has been done in collaboration with Pranjupriya Goswami, Sunder Sahayanathan, Atreyee Sinha, and Ranjeev Misra. Shivappa Bharamappa Gudennavar A stochastic propagation model to the energy de- pendent rapid temporal behaviour of Cygnus X-1 as observed by AstroSat in the hard state We report the results from analysis of six obser- vations of Cygnus X-1 by Large Area X-ray Pro- portional Counter (LAXPC) and Soft X-ray Tele- scope (SXT) onboard AstroSat, when the source was in the hard spectral state as revealed by the broad-band spectra. The spectra obtained from all the observations can be described by a single- temperature Comptonizing region with disc and re- flection components. The event mode data from LAXPC provides unprecedented energy dependent fractional root mean square (rms) and time-lag at different frequencies, which we fit with empirical functions. We invoke a fluctuation propagation model for a simple geometry of a truncated disc with a hot inner region. Unlike other propagation models, the hard X-ray emission ( > 4keV) is as- sumed to be from the hot inner disc by a single- temperature thermal Comptonization process. The fluctuations first cause a variation in the temper- ature of the truncated disc and then the tempera- ture of the inner disc after a frequency dependent time delay. We find that the model can explain the energy dependent rms and time-lag at different fre- quencies. This work has been done in collaboration with Bari Maqbool, M. Sneha Prakash, R. Misra, J. S. Yadav, S. G. Bubbly, A. Rao, S. Jogadand, M. K. Patil, S. Bhattacharyya, and K. P. Singh. Effects of dark matter in star formation The standard model for the formation of structure assumes that there existed small fluctuations in the early universe that grew due to gravitational insta- bility. The origins of these fluctuations are as yet unclear. In this work, we propose the role of dark matter in providing the seed for star formation in the early universe. Very recent observations also support the role of dark matter in the formation of these first stars. With this, we set observable constraints on luminosities, temperatures, and life- times of these early stars with an admixture of dark matter. This work has been done in collaboration with Arun Kenath, A. Prasad, and C. Sivaram. Sarbari Guha Five-dimensional warped product spacetime with time-dependent warping and a scalar field in the bulk We consider gravity in a five-dimensional warped product spacetime, with a time-dependent warp factor and a time-dependent extra dimension. The five-dimensional field equations are derived for a spatially flat FRW brane, and the energy condi- tions and the nature of bulk geometry are exam- ined. It is found that the expansion of the four- dimensional universe depends on its location along the extra dimension, and is different at different lo- cations in the bulk spacetime. At low energies, the trapping of fields within the brane implies a spe- cific correlation between the warp factor and the extra-dimensional scale factor. Generally, the bulk is not conformally flat. At high energies, the bulk is assumed to be sourced by a scalar field with self- interaction. The analysis shows that the potential of the scalar field source of gravity at a given po- sition along the fifth dimension is related to the Hubble parameter on the brane at that position in the bulk. This work has been done in collaboration with Pinaki Bhattacharya. ( 192 )