AR_final file_2018-19

decline. The high brightness and long exposure of the observations enabled to conduct a pulse-phase resolved spectroscopy, where the spectra were ex- tracted corresponding to different phases in the pulse profile, which captures different regions on the surface of NS. He found that the line parame- ters change in tandem with the pulse profile indi- cating the magnetic field is not constant across the surface of the NS but correlates with the flux in the pulse profile. (See Figures 11 and 12). Also, it explains the previously noted asymmetric nature of the cyclotron line as the effect of averaging over the variation in the CRSF over the pulse profile. Core-collapse Supernovae and Super- massive Black Holes Rupak Roy has worked on the disruption of mas- sive stars at the end of their life-time, known as Core-collapse Supernovae (CCSNe), and also on the disruption of stars through tidal forces of the Supermassive Black Holes (SMBHs) at the centers of the galaxies. These are known as Tidal Disrup- tion Events (TDEs). The CCSNe events produce a compact remnant (Neutron star or stellar-mass black hole) due to collapse of the core and the outer material is ejected into space. Recently, astronomers have discovered stellar dis- ruptions, which are 10-100 times more luminous than canonical CCSNe. These are called Superlu- minous Supernovae (SLSNe), the physics of which is still unknown. These extremely energetic phe- nomena (liberated energy ∼ 10 50 ergs ) exist for sev- eral months and are observable mainly in X-ray, UV, and optical wavebands. These are also efficient probes to study the SMBHs in the inactive galaxies. He studied these energetic transients by observing them in optical, UV and X-ray wavelengths using ground and space-based telescopes. Spectro-polarimetric study of two Gamma Ray Bursts Dipankar Bhattacharya , Shabnam Iyyani , and collaborators have been studying the spectro- polarimetric emission of two Gamma Ray Bursts GRB 160802A and GRB 171010A and these have been carried out using simultaneous ob- servations with the AstroSat CZTI and other missions including Fermi and Swift. The avail- ability of polarization measurements place strong constraints on the emission mechanism and view geometry. In GRB 160802A, subphotospheric dissipation appears to dominate the emission, with the observer’s sight line close to the edge of the GRB jet. In GRB 171010A, time variability of polarization indicates a fragmented fireball or ongoing dissipation of Poynting flux. Ionized wind from accretion disk of stellar mass black hole A simultaneous observation by Dipankar Bhat- tacharya , Gulab C. Dewangan and collabora- tors, of an accreting stellar mass black hole 4U 1630-47 with AstroSat, Chandra and MAXI mis- sions has revealed the presence of an ionized wind from the accretion disk, outflowing at about 350 km/s. The broadband spectral shape measured from these observations indicate that the accreting black hole is spinning very rapidly, near the maxi- mum allowed limit. AstroSat timing of a neutron Star The launch of AstroSat was on September 28, 2015 and the LAXPC instrument was turned on and be- came operational on October 19, 2015. On being alerted that 4U 0115+63 was undergoing a major outburst (Ballhausen, et al. 2015), the AstroSat was pointed at this Be/X-ray binary pulsar, and observations were made with the LAXPC instru- ment on October 24, 2015 during the peak of a giant type II outburst. Prominent intensity os- cillations (quasi-periodic oscillations) at ∼ 1 and ∼ 2 mHz frequency were detected in LAXPC and Nuclear Spectroscopic Telescope Array (NuSTAR) observations during this outburst by Jayashree Roy , and collaborators (see Figure 8). The slow- est 1mHz oscillation detected from this source in transient X-ray binary pulsars was detected upto 50 keV by LAXPC. Millihertz oscillations from this source are also observed from Rossi X-ray Timing ( 74 )