Hard X-ray polarimetry with AstroSat-CZTI: an overview of the technique and the scientific findings

CZT Imager onboard AstroSat is primarily a hard X-ray spectroscopic instrument but provides sensitive polarization measurements above 100 keV for bright X-ray sources, e.g. Crab pulsar and Cygnus X-1 black hole system. With an accumulated 800 ks of observation, CZTI provided till date statistically the most significant polarization measurement for Crab. Phase-resolved polarimetry of the pulsed radiation of Crab was also attempted for the first time in X-rays and the findings provided interesting insights to the emission site of hard X-rays. In the case of Cygnus X-1, the recent dedicated long AstroSat observations show a strong dependence of polarization properties on the spectral state of the source. This is an interesting finding and might be useful in understanding possible disk-jet interplay in black hole systems. Another important characteristic of CZTI is the increasing transparency of the support structure in hard X-rays, which enables it to function as a wide field hard X-ray monitor providing an unique opportunity for GRB detection and polarization measurement. From a sample of a large number of GRBs detected by CZTI in the last 5 years, we attempted polarization measurement for 20 bright GRBs. Time integrated analysis of the prompt emission yields low or no polarization indicating that GRB prompt emission is highly structured leading to possible changes in the polarization angle within a burst which has also been seen independently for some of the bright AstroSat GRBs. In this presentation, I would briefly discuss these recent scientific findings from CZTI. Polarization measurement in hard X-rays for CZT based large area instruments is challenging because of multiple factors, e.g. large background, difficulty in properly reconstructing the Compton events in a single crystal detector plane, systematic effects because of square pixel geometry and the effect of charge sharing. Polarization measurements also critically depend on the accuracy of the mass model of the instrument and the spacecraft particularly for off-axis sources like GRBs. We inspected each of these effects carefully and developed our analysis algorithm over the last few years. Some of these challenges and their effects will be highlighted in the presentation in the context of CZTI polarimetry technique.