AR_final file_2018-19

could infer about the nature of the accretion disk, which has been, till date, done only by observ- ing the electro-magnetic signals coming from it. This multi-messenger astronomy was for the first time conceived and worked out during our research work. Piyali Bhar Anisotropic compact star model: A brief study via embedding In the present work, a new model of compact star is obtained in the framework of general relativity, which does not suffer from any kinds of singular- ity. We assume that the underlying fluid distri- bution is anisotropic in nature along with a new form for the metric potential e λ , which is phys- ically reasonable. Though the model parameters depend on four constants a, b, A and B , but we have shown that the solutions depend on two free constants since these four constants are correlated to one another. Our proposed model of anisotropic compact star obeys all the necessary physical re- quirements, which have been analyzed with the help of the graphical representation, where n lies in the range of − 200 ≤ n ≤ 200. We have shown that the model satisfies all the energy conditions as well as the causality condition. The model is po- tentially stable and also satisfy Harrison-Zeldovich- Novikov’s stability condition. Naseer Iqbal Bhat Log-normal flux distribution of bright Fermi blazars We present the results of the γ -ray flux distribu- tion study on the brightest blazars, which are ob- served by the Fermi-LAT. We selected 50 brightest blazars based on the maximum number of detec- tion reported in the LAT third AGN catalog. We performed standard unbinned maximum likelihood analysis on the LAT data during the period be- tween August 2008 and December 2016, in order to obtain the average monthly flux. After qual- ity cuts, balazars for which at least 90% of the to- tal flux was survived were selected for the further study, and this includes 19 FSRQs and 19 BL Lacs. The Anderson-Darling and χ 2 tests suggest that the integrated monthly flux follow a log-normal dis- tribution for all sources, except for three FSRQs for which neither a normal nor a log-normal distribu- tion was preferred. A double log-normal flux dis- tribution tendency were observed in these sources, though it has to be confirmed with improved statis- tics. We also found that the standard deviation of the log-noraml flux distribution increases with the mean spectral index of the blazar, and can be fitted with a line of slope 0 . 24 ± 0 . 04. We repeat our study on three additional brightest unclassified blazars to identify their flux distribution properties. Based on the features of their log-normal flux dstribution, we infer these unclassified blazars may be closely as- sociated with FSRQs. We also highlight that con- sidering the log-normal behaviour of the flux dis- tribution of blazars, averaging their long term flux in linear scale can largely under-estimate the nor- mal flux, and this discrepancy can propagate down to the estimation of the source parameters through spectral modelling. This work has been done in collaboration with Zahir Shah, N. Mankuzhiyil, A. Sinha, Ranjeev Misra, and S. Sahayanathan. Study on temporal and spectral behaviour of 3C 279 during 2018 January flare We present a detailed temporal and spectral study of the blazar 3C 279 using multi-wavelength obser- vations from Swift -XRT, Swift -UVOT, and Fermi - LAT during a flare in 2018 January. The tempo- ral analysis of γ -ray light curve indicates a lag of ∼ 1 d between the 0 . 1 − 3 GeV and 3 − 500 GeV emission. Additionally, the γ -ray light curve shows asymmetry with slow rise-fast decay in energy band 0 . 1 − 3 GeV and fast rise-slow decay in the 3 − 500 GeV band. We interpret this asymmetry as a result of shift in the Compton spectral peak. This inference is further supported by the correla- tion studies between the flux and the parameters of the log-parabola fit to the source spectra in the energy range 0 . 1 − 500 GeV . We found that the flux corrrelates well with the peak spectral energy and the log-parabola fit parameters show a hard index with large curvature at high flux states. In- terestingly, the hardest index with large curvature was synchronous with a very high energy flare de- tected by H.E.S.S. Our study of the spectral be- haviour of the source suggests that γ -ray emission is most likely to be associated with the Compton up scattering of IR photons from the dusty envi- ronment. Moreover, the fit parameters indicate the increase in bulk Lorentz factor of emission region to be a dominant cause for the flux enhancement. This work has been done in collaboration with Za- hir Shah, V. Jitesh, S. Sahayanathan and Ranjeev ( 176 )