Abstract Details

Affiliation: IUCAA, Pune
Conference ID: ASI2017_548
Title : AstroSat/LAXPC hard X-ray view of Galactic micro-quasars : A new window to explore hard X-ray variability above 20 keV
Authors and Co-Authors : Ranjeev Misra (IUCAA, Pune), J S Yadav (TIFR, Mumbai), H M Antia (TIFR, Mumbai), Jai Verdhan Chauhan (TIFR, Mumbai)
Abstract Type : Poster
Abstract Category : Stars,ISM and the Galaxy
Abstract : Owing to the higher efficiency than RXTE/PCA (by an order of magnitude above 10 keV) and unprecedented time resolution (~ 10 micro-seconds), Large Area X-ray Proportional Counter (LAXPC) on-board AstroSat provided a rare opportunity to study hard X-ray spectro-timing variability of Galactic micro-quasars up to 80 keV which was never satisfactorily performed by any of the contemporary or past X-ray space missions for astronomical observations. In this presentation, I shall focus on results obtained from the hard X-ray variability study of four Galactic micro-quasars: GRS 1915+105, Cyg X-1, Cyg X-3 and H 1743-322 which were extensively monitored by LAXPC during last one year. Such studies provide valuable information about the source accretion properties in the hard X-ray regime at different spectral states, namely hard/hard-intermediate and soft/soft-intermediate. Till date, many attempts have been made to connect spectral and temporal variability in these micro-quasars but unfortunately most of them are limited to 20 keV which is the upper threshold with the most reliable calibration in RXTE/PCA. With the launch of AstroSat, LAXPC reliably extends the higher energy limit to 80 keV. This allows us to test and extend our knowledge of source spectral state evolution from the soft X-ray band to the hard X-ray band. For example, spectral analysis showed existence of high energy cut-off in few source between 20-80 keV. However, spectral fitting suffers from degeneracy problem. On the other way, tracking source evolution in the hardness intensity diagram is a model-independent approach. Therefore, 'Can we use hardness intensity diagram to detect spectral cut-off/pivoting ?' Answer to this question was not possible with RXTE/PCA but it is possible with AstroSat observations. Secondly, quasi-periodic oscillation (QPO) frequency from GRS 1915+105 are reported to increase with photon energy up to 20 keV. Such observations can discard many QPO model but the confirmation on such event requires independent observation which LAXPC can provide reliably. Among many interesting aspects of high energy spectro-temporal behaviour, these are the key results which I shall discuss. I shall also discuss the importance of the hard X-ray spectro-temporal variability study in understanding the accretion and radiation mechanism in Galactic micro-quasars and how it can serve as an excellent diagnostic for the accretion around black holes.