30th Annual Report 2017-18

33 Some of the initial sources observed by AstroSat were well known black hole systems, in order to demonstrate AstroSat's capabilities and reveal the rapid variability properties of these sources in hard X-ray energies which had not been seen before. Ranjeev Misra and Mayukh Pahari as part of the national level LAXPC Science Team, have analysed the complex data from such sources. Data from the well known bright black hole source GRS 1915+105, revealed that the source was on kind of variability state which shows quasi- periodic oscillations (i.e., oscillations which are like but not exactly sinusoidal, QPOs) in the frequency range of 3 to 6 Hz. For these oscillations, they reported the arrival time difference between the very high energy X-ray photons (30 keV) and the lower 5 keV ones. Since higher energy photons are expected to be produced closer to the black hole than low energy ones, such analysis provides unique clues to the geometry of the inner accretion disk and the nature of spacetime near the black hole. This was the first science result reported byAstroSat and it heralds a new era of X-ray timing studies. This was followed by analysis of another well known black hole system Cygnus X-1. This source does not show quasi-periodic oscillations, but instead its variability is characterized by broad band features. The LAXPC Science Team which included Ranjeev Misra and Mayukh Pahari , used AstroSat/LAXPC data to study the variability in high energies for the first time. They were able to measure the time difference between different energy photons to a precision level not achieved before. Moreover, using the event mode data, they could create spectra at different fluxes varying on time-scales of 1 second, which was not possible earlier. The spectra at different fluxes showed steepening at higher fluxes which meant that the variability in different energy bands are driven by the low energy photons, i.e., photons from the outer disk. One of the important capabilities of the LAXPC instrument is the detection of high frequency phenomenon from compact object. Ranjeev Misra and other members of the LAXPC team were the first to demonstrate millisecond variability measurement capability of LAXPC by detecting a 830 Hz , Quasi-periodic Oscillations (QPOs) in the time variability of the neutron star system 4U 1728-34. While such QPOs have been detected earlier by RXTE, the novelty here is, its detection above energies of 10 keV, which is due to larger effective area of LAXPC. Neutron stars undergo thermo-nuclear burning on their surfaces which are observed as short 10 second burst. During one of the bursts of 4U 1728-34, a burst oscillation at 363 Hz which is identified with the spin period of the neutron star was detected by LAXPC. Figure 3: Time lag in milliseconds between photons of 20-40 keVw.r.t. to those of 5-10 keV energies. Such detailed informtion at these high energies was made possible byAstroSat LAXPC. (Figure taken from Misra , et al. 2017,ApJ, 835, 195).