30th Annual Report 2017-18

Figure 1. Phase-resolved polarisation measurement of Crab Nebula and the Pulsar with AstroSat/CZTI. Left panel: The grey line shows the brightness of the Crab Pulsar as observed byAstroSat CZTI. The horizontal axis (phase) represents time expressed in units of the pulsar's spin period. Phase 0.0 to 1.0 stands for the full rotation cycle of the pulsar. The same result is shown repeated between phase 1.0 and 2.0, for a clear demonstration of the periodic pattern. Coloured bars indicate how strongly polarised the observed radiation is. Sharp variation of polarisation, when the brightness is, low is the surprising discovery by AstroSat. Right panel: The angle of X-ray polarisation measured by AstroSat CZTI shown superposed on a composite optical and X-ray image of the Crab Nebula, taken by NASA's Hubble and Chandra telescopes respectively. The white arrow represents the projected spin axis of the pulsar located at the centre of the nebula. Other arrows display the orientation of the observed polarisation. The colour of an arrow indicates the range of phase it belongs to, being equal to that spanned by bars of the corresponding shade in the left panel. X-ray Binaries withAstroSat/LAXPC The LAXPC onboard AstroSat is superior to its predecessor on-board the American satellite "Rossi X-ray Timing Experiment (RXTE)" by having a significantly larger area in high energies (> 20 keV) and by having event mode data wherein each photon information is recorded leading to great flexibility in analysis. Combined with the other instruments on board, namely the Soft X-ray telescope (SXT), the Ultra Violet Imaging Telescope (UVIT) and the Cadmium Zinc Telluride Imager (CZTI) makes LAXPC the ideal instrument to unravel themysteries of rapidly varying bright X-ray sources such as black hole and neutron star systems. Figure 2: The power spectrum (left) showing the 2.5 Hz QPO for the black hole systemGRS 1915+105. The middle and right panels show the time lag and r.m.s. as a function of energy. Results for energy greater than 20 keV were obtained for the first time using AstroSat LAXPC. (Figure taken from Yadav, et al. 2017,ApJ, 833, 27). 32