Hard X-ray Polarimetry with AstroSat CZTI

S. V. Vadawale, A. R. Rao, D. Bhattacharya, T.Chattopadhyay, Mithun, N. P. S., V. Bhalerao, A. Vibhute, S. Sreekumar + CZTI Team

The Cadmium Zinc Telluride Imager (CZTI) is a wide-field coded aperture mask instrument on AstroSat, primarily designed for simultaneous hard X-ray imaging and spectroscopy in the energy range of 20 to 150 keV.  The detector plane of CZTI consists of a large array of pixilated CZT detectors with total 16384 pixels, each having size of 2.5 × 2.5 mm2 and thickness of 5 mm. The larger thickness and the fine pixilated nature of the detector plane of CZTI is also helpful in extending useful energy range to ~250 keV as well as to measure polarization in hard X-rays, by identifying the X-ray photons interacting by means of Compton scattering. As an added advantage of the extended energy range, CZTI is also sensitive to hard X-ray transients like gamma-ray bursts.  CZTI has been operating successfully since the launch of AstroSat and has detected many bright X-ray sources. Apart from the regular imaging and spectroscopic measurements, CZTI has successfully measured hard X-ray polarization in a few sources including the standard candle source, the Crab nebula; a black hole binary Cygnus X-1 and a number of gamma-ray bursts. Here I will present the early results of the hard X-ray polarization measurements with the AstroSat CZTI carried out with observations during the performance verification phase and guaranteed time observations.