AR-2019-2020

Umananda Dev Goswami Supersymmetric hybrid inflation with non-minimal coupling to gravity The paradigm of cosmic inflation has been stud- ied using the simplest model based on the idea of supersymmetric hybrid inflation with non-minimal coupling to gravity, specially under the slow-roll approximation following the superconformal ap- proach to supergravity. It is found that within a range of values of the non-minimal coupling param- eter ξ , the model can accommodate the inflation data reported by the Planck ( n s and upper limit of r ) and BICEP2/Keck (upper limit of r ) collab- orations. The study shows that the most probable value of ξ should be ∼ 0 . 0134 ± 0 . 0005. That is cou- pling is found to be very weak. Within this range of ξ , the values of r estimated from our model for 50 − 70 e-foldings are found to lay well below the upper limits set by the Planck and BICEP2/Keck collaborations. Similarly, values of n s obtained for the said parameters are in good agreement with its latest data of the Planck collaboration. The con- straint equations for running of the scalar spectral index n sk and its running n skk are derived from the numerical solutions of our model for these parame- ters. These equations can be used to test our model from the data of future cosmological observations. Shivappa Bharamappa Gudennavar Are narrow-line Seyfert 1 galaxies powered by low- mass black holes? Narrow-line Seyfert 1 galaxies (NLS1s) are believed to be powered by the accretion of matter onto low- mass black holes (BHs) in spiral host galaxies with BH masses M BH ∼ 10 6 to 10 8 M . However, the broadband spectral energy distribution of the γ - ray-emitting NLS1s are found to be similar to flat- spectrum radio quasars. This challenges our cur- rent notion of NLS1s having low M BH . To resolve this tension of low M BH values in NLS1s, we fit- ted the observed optical spectrum of a sample of radio-loud NLS1s (RL-NLS1s), radio-quiet NLS1s (RQ-NLS1s), and radio-quiet broad-line Seyfert 1 galaxies (RQ-BLS1s) of ∼ 500 each with the stan- dard Shakura-Sunyaev accretion disk (AD) model. For RL-NLS1s, we found a mean log( M AD BH /M ) of 7.98 ± 0.54. For RQ-NLS1s and RQ-BLS1s, we found mean log( M AD BH /M ) of 8.00 ± 0.43 and 7.90 ± 0.57, respectively. While the derived M AD BH val- ues of RQ-BLS1s are similar to their virial masses, for NLS1s the derived M AD BH values are about an order of magnitude larger than their virial esti- mates. Our analysis, thus indicates that NLS1s have M BH similar to RQ-BLS1s, and their avail- able virial M BH values are underestimated, influ- enced by their observed relatively small emission line widths. Considering Eddington ratio as an es- timation of the accretion rate and using M AD BH , we found the mean accretion rate of our RQ-NLS1s, RL-NLS1s, and RQ-BLS1s as 0 . 06 +0 . 16 − 0 . 05 , 0 . 05 +0 . 18 − 0 . 04 and 0 . 05 +0 . 15 − 0 . 04 , respectively. Our results therefore, suggest that NLS1s have BH masses and accretion rates that are similar to BLS1s. This work has been done in collaboration with Gayathri Viswanath, C. S. Stalin, Suvendu Rakshit, Kshama S. Kurian, K. Ujjwal, et al. Unveiling the temporal properties of MAXI J1820+070 through AstroSat observations We present here the results of the first broadband simultaneous spectral and temporal studies of the newly detected black hole binary MAXI J1820+070 as seen by Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) on board AstroSat. The observed combined spectra in the energy range 0.7-80 keV were well modelled using disk blackbody emission, thermal Comptonization, and a reflection component. The spectral analy- sis revealed that the source was in its hard spectral state (Γ = 1 . 61) with a cool disk ( kT in = 0 . 22 keV ). We report the energy dependent time-lag and root mean squared (rms) variability at different frequen- cies in the energy range 3-80 keV using LAXPC data. We also modelled the flux variability using a single-zone stochastic propagation model to quan- tify the observed energy dependence of time lag and fractional rms variability, and then compared the results with that of Cygnus X-1. Additionally, we confirm the detection of a quasi-periodic oscilla- tion with the centroid frequency at 47.7 mHz. This study has been done in collaboration with Sneha Prakash Mudambe, Bari Maqbool Bhat, Ranjeev Misra, Sabhya Hebbar, J. S. Yadav, et al. Sarbari Guha Thermodynamics of FRW universe with Chaplygin gas models In this work, we have examined the validity of the generalized second law of thermodynamics (GSLT)