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 the99%confidence minimum detectable polarization levels of72.3 ± 0.8%(SM) and57.5 ± 0.8%(MLM). Therefore, COSI did not detect polarized gamma-ray emission from this burst. Our most constraining90%confidence upper limit on the polarization level was46%(MLM).