Polarimetric Analysis of the Long Duration Gamma Ray Burst GRB 160530A With the Balloon Borne Compton Spectrometer and Imager

TL;DR

This study used the COSI balloon instrument to measure gamma-ray polarization from GRB 160530A, but found no significant polarization, setting an upper limit of 46%.

Contribution

First polarization measurement attempt of a long gamma-ray burst with COSI, demonstrating the instrument's capabilities and limitations.

Findings

No significant polarization detected in GRB 160530A

Set a 90% confidence upper limit of 46% on polarization

Established sensitivity thresholds for future gamma-ray polarization studies

Abstract

A long duration gamma-ray burst, GRB 160530A, was detected by the Compton Spectrometer and Imager (COSI) during the 2016 COSI Super Pressure Balloon campaign. As a Compton telescope, COSI is inherently sensitive to the polarization of gamma-ray sources in the energy range 0.2-5.0 MeV. We measured the polarization of GRB 160530A using 1) a standard method (SM) based on fitting the distribution of azimuthal scattering angles with a modulation curve, and 2) an unbinned, maximum likelihood method (MLM). In both cases, the measured polarization level was below the $99\%$ confidence minimum detectable polarization levels of $72.3 \pm 0.8\%$ (SM) and $57.5 \pm 0.8\%$ (MLM). Therefore, COSI did not detect polarized gamma-ray emission from this burst. Our most constraining $90\%$ confidence upper limit on the polarization level was $46\%$ (MLM).