Exploring hard X-ray polarimetry with Astrosat-CZTI

The CZT Imager instrument onboard recently launched Astrosat is a hard X-ray telescope capable of spectroscopy and imaging in the energy range of 20 - 150 keV using the technique of coded mask imaging. It consists of a large array of pixilated CZT detector modules as the detector plane having total geometric area of 976 cm^{2} and thickness of 5 mm. Since the detectors have significant detection efficiency beyond the primary energy range of CZTI, it can also be used for spectroscopy as well as polarization measurement in the extended energy range of 150 - 300 keV by identifying the Compton scattering events in the adjacent pixels. The polarimetric capability of Astrosat-CZTI using the Compton events has been investigated in detail by means of Geant4 simulations and has been experimentally demonstrated during the ground testing of CZTI. The pertinence of Compton events in the CZTI detector plane for astrophysical observation has demonstrated during the early observation with the detection of multiple GRBs in the Compton events. One GRB detected by CZTI show presence of azimuthal modulation suggesting highly polarized nature of the X-ray emission. The initial Crab observations also indicate presence of azimuthal modulation. Here we present preliminary analysis of the Compton events in the multiple observations carried out during the initial performance verification phase of Astrosat CZTI.