Design optimisation and testing of SSM on ASTROSAT and study of X-ray transients

Abstract

Astrosat is India’s first multiwavelength mission dedicated for astronomy to do observa- tions of different classes of galactic and extragalactic sources in the spectral band of optical to hard X-rays. ASTROSAT is planned for launch in 2011 with five instruments on-board. One of the instruments is the Scanning Sky Monitor (SSM). SSM is an X-ray sky monitor which scans the entire sky in few hours to detect and locate any X-ray transients occur- ring in its Field Of View (FOV), in the energy range 2 to 10 keV. SSM has a large FOV and imaging of the X-ray sky is done using a coded-mask on the detector. SSM detectors are position-sensitive gas proportional counters. SSM comprises of three identical detec- tors with the associated electronics for each detector. A coded-mask with a collimator is mounted on top of each detector unit which defines a slightly different FOV for each unit. The three SSM units are mounted on a single rotating platform to scan the sky. The thesis work presented here includes design optimisation of certain parameters of the gas proportional counters like the cell size, the gas mixture, gas pressure etc. for better performance of SSM detectors to achieve the science objectives. Characterisation of SSM detectors for stability of the output, detection efficiency in the energy range of interest, gas gain at different operating voltage, energy resolution, position resolution etc. are exper- imentally carried out and the results are discussed. As per the science objective of SSM to detect and locate any X-ray source in its FOV, it is required that the position of every photon incident on SSM detector is derived. The methodology to derive the position of the photon incident on SSM is devised and verified. Position calibration of SSM detectors are carried out experimentally and the anode calibration constants are derived. Various experiments are carried out to verify the calibration constants. Experiment to locate the position of the X-ray source in the FOV of SSM is carried out and found successful. Energy calibration of SSM at different energies in the energy range of interest is done and the spec- tral response of SSM is generated. The on-board calibration procedure for SSM is planned using the Crab nebula. Using the spectral response for SSM, the pulse height spectrum of the Crab nebula is simulated and the Crab count for SSM is estimated. SSM detectors can detect and locate the transient X-ray sources, but are not meant for detailed spectral observations. A more detailed observation of these X-ray transient sources are carried out using pointed-mode instruments which have a narrow FOV and observe the sources for a longer duration. Two X-ray transient sources observed by the RXTE mission are studied for spectral and timing properties. The details of the analysis are presented in this thesis. The two X-ray transients studied here are SLX 1746-331 and Swift J1753.5-0127. The mass estimate for these sources indicates them to be black hole candidates.