The proposed Daksha mission comprises of a pair of highly sensitive space telescopes for detecting and characterizing high-energy transients, such as electromagnetic counterparts of gravitational wave events and gamma-ray bursts (GRBs). Along with spectral and timing analysis, Daksha can also undertake polarization studies of these transients, providing data crucial for understanding the source geometry and physical processes governing high-energy emission. Each Daksha satellite will have 340 pixelated cadmium zinc telluride (CZT) detectors arranged in a quasi-hemispherical configuration without any field-of-view collimation (open detectors). These CZT detectors are good polarimeters in the energy range 100 to 400 keV, and their ability to measure polarization has been successfully demonstrated by the cadmium zinc telluride imager onboard AstroSat. Here, we demonstrate the hard x-ray polarization measurement capabilities of Daksha and estimate the polarization measurement sensitivity (in terms of the minimum detectable polarization: MDP) using extensive simulations. We find that Daksha will have MDP of 30% for a fluence threshold of 10 − 4 erg cm − 2 (in 10 to 1000 keV). We estimate that with this sensitivity, if GRBs are highly polarized, Daksha can measure the polarization of about five GRBs per year.
The cadmium zinc telluride imager (CZTI) on board AstroSat consists of an array of a large number of pixelated cadmium zinc telluride (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 to 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 such as 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 pixelated CZT detectors through controlled experiments carried out with a CZT detector similar to that used in CZTI and extensive Geant4 simulations of the experimental setup. Our current results show that the CZT detectors can be used to measure the polarization of bright GRBs with off-axis angles of up to ∼60 deg. However, at incidence angles between 45 deg and 60 deg, there might be some systematic effects that need to be taken into account when interpreting the measured polarization fraction.
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