Experimental verification of off-axis polarimetry with Cadmium Zinc Telluride detectors of AstroSat-CZT Imager

TL;DR

This paper experimentally verifies the off-axis polarimetric capabilities of CZT detectors used in AstroSat-CZTI, demonstrating their effectiveness for measuring polarization of bright GRBs up to 60 degrees off-axis with some systematic considerations.

Contribution

The study provides the first experimental calibration of off-axis polarization sensitivity of CZT detectors, supported by Geant4 simulations, enhancing the reliability of polarization measurements for off-axis sources.

Findings

CZT detectors can measure polarization of bright GRBs up to ~60° off-axis.

Systematic effects are present at 45-60° incidence angles.

Experimental results align with Geant4 simulations.

Abstract

The Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat consists of an array of a large number of pixellated CZT detectors capable of measuring the polarization of incident hard X-rays. The polarization measurement capability of CZTI for on-axis sources was experimentally confirmed before the launch. CZTI has yielded tantalizing results on the X-ray polarization of the Crab nebula and pulsar in the energy range of 100 - 380 keV. CZTI has also contributed to the measurement of prompt emission polarization for several Gamma-Ray Bursts (GRBs). However, polarization measurements of off-axis sources like GRBs are challenging. It is vital to experimentally calibrate the CZTI sensitivity to off-axis sources to enhance the credence of the measurements. In this context, we report the verification of the off-axis polarimetric capability of pixellated CZT detectors through the controlled experiments carried out with a CZT detector similar to that used in CZTI and extensive Geant4 simulations of the experimental set-up. Our current results show that the CZT detectors can be used to measure the polarization of bright GRBs up to off-axis angles of ~60 degrees. However, at incidence angles between 45-60 degrees, there might be some systematic effects which needs to be taken into account while interpreting the measured polarisation fraction.