AstroSat CZT Imager observations of GRB 151006A: timing, spectroscopy, and polarisation study

TL;DR

This paper demonstrates AstroSat's CZT Imager capability to localize, analyze spectra, and detect polarization in gamma-ray bursts, enhancing multi-instrument observations and providing new insights into GRB properties.

Contribution

The study shows CZT Imager's ability to localize GRBs, match spectral parameters of existing instruments, and identify polarization, especially for bright GRBs, adding new observational capabilities.

Findings

CZT Imager can localize GRBs within a few degrees.

It provides spectral parameters comparable to Fermi/GBM.

It can detect polarization signals in bright GRBs.

Abstract

AstroSat is a multi-wavelength satellite launched on 2015 September 28. The CZT Imager of AstroSat on its very first day of operation detected a long duration gamma-ray burst (GRB) namely GRB 151006A. Using the off-axis imaging and spectral response of the instrument, we demonstrate that CZT Imager can localise this GRB correct to about a few degrees and it can provide, in conjunction with Swift, spectral parameters similar to that obtained from Fermi/GBM. Hence CZT Imager would be a useful addition to the currently operating GRB instruments (Swift and Fermi). Specifically, we argue that the CZT Imager will be most useful for the short hard GRBs by providing localisation for those detected by Fermi and spectral information for those detected only by Swift. We also provide preliminary results on a new exciting capability of this instrument: CZT Imager is able to identify Compton scattered events thereby providing polarisation information for bright GRBs. GRB 151006A, in spite of being relatively faint, shows hints of a polarisation signal at 100-300 keV (though at a low significance level). We point out that CZT Imager should provide significant time resolved polarisation measurements for GRBs that have fluence 3 times higher than that of GRB 151006A. We estimate that the number of such bright GRBs detectable by CZT Imager is 5 - 6 per year. CZT Imager can also act as a good hard X-ray monitoring device for possible electromagnetic counterparts of Gravitational Wave events.