Prompt Emission Polarimetry of Gamma-Ray Bursts with the AstroSat CZT Imager

Abstract

X-ray and Gamma-ray polarization measurements of the prompt emission of Gamma-ray bursts (GRBs) are believed to be an important tool to test the various models of GRBs. Although there are some reports of hard X-ray polarization measurements of the prompt emission of GRBs, the number of measurements are small to provide statistically significant inputs to the GRB models due to the extreme difficulty of measuring them and quantifying their significance. CZTI onboard AstroSat is primarily an X-ray spectroscopic instrument but works as a wide angle GRB monitor due to the increasing transparency of the CZTI support structure. It also has experimentally verified polarization measurement capability in the 100 − 300 keV energy range and thus provides a unique opportunity to attempt spectro-polarimetric studies of GRBs. Here we present the polarization data for the brightest 11 GRBs detected by CZTI during the first year of operation. Most of the GRBs show clear polarization signatures with ≥3σ detection significance for 4 GRBs and ∼2.5σ significance for another 3 GRBs. We could place meaningful upper limits for the remaining 4 GRBs. We provide the details of the various tests performed to validate the polarization measurements. While it is difficult to differentiate the various emission models with the current sample of polarization measurements, CZTI in its minimum lifetime of five years is expected to provide a large sample of polarization measurements which would lead to a better understanding of the prompt emission.

Time-resolved GRB polarization with POLAR and GBM

“…To enable performing joint fits of the spectra and the polarization a novel analysis technique was developed. Traditional polarization analysis techniques, such as those employed in Yonetoku et al 2011, Chattopadhyay et al (2017) as well as in Zhang et al (2019), rely on fitting data to responses produced for a specific spectrum. This method does not allow joint fits of both the spectrum and polarization parameters, nor does it allow naturally including systematic uncertainties from the spectral fits into the systematic uncertainties of the polarization.…”

“…We stress that it is not appropriate to perform model comparison on nested model parameters, for example, comparing between zero polarization and greater than zero polarization. This includes the use of Bayes factors (Chattopadhyay et al 2017) which are ill-defined for improper priors and for comparing between discrete values of a continuous parameter (Gelman et al 2013). Polarization is not a detected quantity, but a parameter.…”

“…This indicates the importance of performing polarization measurements with carefully calibrated and dedicated instrumentation. More recently, the AstroSAT collaboration has reported preliminary polarization analysis results of several GRBs (Chattopadhyay et al 2017). The systematics and procedures related to obtaining these measurements is not immediately clear.…”

Spectropolarimetric analysis of prompt emission of GRB 160325A: jet with evolving environment of internal shocks

“…Linear polarisation measurement of the prompt emission of GRBs is a powerful tool, in addition to spectral analysis, to strongly constrain the radiation process, geometry of the emitting region and the composition of the GRB outflow (Covino & Gotz 2016). Currently, polarisation measurement of prompt emission has been reported for only a handful of cases (Wigger et al 2004;Willis et al 2005;McGlynn et al 2007;Götz et al 2009;McGlynn et al 2009;Yonetoku et al 2011Yonetoku et al , 2012Götz et al 2013Götz et al , 2014Zhang et al 2019;Burgess et al 2019b;Chand et al 2018Chand et al , 2019Sharma et al 2019;Chattopadhyay et al 2019; for a recent review see McConnell 2017). Linear polarisation has also been measured in early optical afterglows of a few GRBs (Wijers et al 1999;Covino et al 2002;Greiner et al 2003;King et al 2014;Gorbovskoy et al 2016;Troja et al 2017;Laskar et al 2019).…”

X-ray imaging polarimetry with a 2.5-μm pixel CMOS sensor for visible light at room temperature

“…8 Polarization of Cyg X-1 has also been measured with some of these instruments. Gamma-ray bursts are also the targets of extensive observations of their polarization, with GAP on IKAROS, 9 the CZT Imager on ASTROSAT, 10 and POLAR on-board Tiangong-2. 11 The imaging x-ray polarimetry explorer (IXPE), 12 a small satellite mission dedicated to soft x-ray polarimetry, is scheduled to be launched in 2021.…”