Hard X-Ray Polarization Catalog for a Five-year Sample of Gamma-Ray Bursts Using AstroSat CZT Imager

Abstract

The Cadmium Zinc Telluride Imager (CZTI) on board AstroSat has been regularly detecting gamma-ray bursts (GRBs) since its launch in 2015. Its sensitivity to polarization measurements at energies above 100 keV allows CZTI to attempt spectropolarimetric studies of GRBs. Here, we present the first catalog of GRB polarization measurements made by CZTI during its first five years of operation. This includes the time-integrated polarization measurements of the prompt emission of 20 GRBs in the energy range 100–600 keV. The sample includes the bright GRBs that were detected within an angle range of 0°–60° and 120°–180° where the instrument has useful polarization sensitivity and is less prone to systematics. We implement a few new modifications in the analysis to enhance the polarimetric sensitivity of the instrument. The majority of the GRBs in the sample are found to possess less/null polarization across the total bursts’ duration in contrast to a small fraction of five GRBs that exhibit high polarization. The low polarization across the bursts might be due either to the burst being intrinsically weakly polarized or to a varying polarization angle within the burst even when it is highly polarized. In comparison to POLAR measurements, CZTI has detected a larger number of cases with high polarization. This may be a consequence of the higher energy window of CZTI observations, which results in the sampling of a shorter duration of burst emissions than POLAR, thereby probing emissions with less temporal variation in polarization properties.

Experimental verification of off-axis polarimetry with cadmium zinc telluride detectors of AstroSat-CZT Imager

“…We also verified that both the modulation curve fitting method and template fitting method yield similar results, particularly when the number of Compton events is limited as in case of CZTI GRBs. This further validates the methods and results reported in Chattopadhyay et al (2019) 37 and Chattopadhyay et al (2022) 38 for GRB prompt emission.…”

“…In the analysis of experimental ASADs, we had to account for charge sharing effect. Chattopadhyay et al (2022), 38 for a sample of twenty bright GRBs, showed that the ratio of Compton to Single Pixel events is consistently higher in experimental data than in simulation results (see Figure 1 of Chattopadhyay et al, 2022 38 ). This effect is attributed to charge sharing between the adjacent pixels which can mimic a Compton event and consequently add to the number of true Compton events.…”

“…(2022). 38 These corrected simulation ASADs are used in further polarimetric analysis of experimental ASADs.…”

“…This modulation curve fitting approach is followed in the polarimetric analysis of GRBs presented in Chattopadhyay et al (2019) 37 and Chattopadhyay et al (2022). 38 While the modulation curve obtained by normalizing with the unpolarized ASAD allows easier visualization of the presence of polarization signature, studies have shown that inference of PF and PA by modulation curve fitting is prone to systematic effects when used for off-axis X-ray polarimetry. 46 Muleri (2014) has shown that the geometry corrected ASAD does not follow a cosine distribution when the photons are incident at off-axis angles as the ASAD samples different ranges of polar scattering angles.…”

“…35,36 CZTI measured the polarization of 11 GRBs during its first year of operation. 37 Recently, Chattopadhyay et al(2022) 38 presented a catalog of polarization measurements of 20 GRBs selected with more stringent criteria on fluence and minimum detectable polarization from all the GRBs observed by CZTI during first five years of its operation. GRB polarimetry of such a large sample with definite polarization measurements would help distinguish between various prompt emission models.…”