Gamma-ray bursts (GRBs) are highly luminous flashes of gamma-rays that last from hundreds of milliseconds up to hundreds of seconds, peak in the soft gamma-ray band 0.1 MeV to 1 MeV, represent ultra-relativistic motion, and originate from cosmological distances. Despite thousands of observations of GRB prompt emission, and hundreds of observations of GRB afterglows, many details pertaining to the nature of gamma-ray bursts remain elusive. A wide variety of models exist that describe the progenitors, central engines, emission mechanism, geometric structure, and magnetization of GRBs. Polarization measurements of the GRB prompt emission are believed to give essential clues where spectral and temporal observations fall short.
The Compton Spectrometer and Imager (COSI) is a balloon borne Compton polarimeter, imager, and spectrometer sensitive in the 0.2 MeV to 5 MeV band. One of COSI's main science goals is measuring the polarization of astrophysical gamma-ray sources, including GRBs. Polarimetric measurements with a Compton polarimeter are possible due to the sensitivity of the Compton scattering cross section upon the orientation of the photon's electric field vector. We verified COSI's polarimetric capabilities in the laboratory using partially polarized and unpolarized beams, and compared the measurements to simulations. Overall, the measurements reflect the simulations closely, and we were able to place upper limits on the systematic error on the polarization level of 3% - 4%.
In May of 2016, COSI took flight from Wanaka, New Zealand aboard a NASA Super Pressure Balloon. COSI continuously telemetered science data for 46 days before landing in Southern Peru. On May 30, 2016, COSI clearly detected the long duration gamma-ray burst GRB 160530A. We performed a polarization analysis of this GRB using both a standard method (SM) and a maximum likelihood method (MLM). The measured polarization level was below the 99\% confidence minimum detectable polarization level (MDP) using both analysis methods (MDP = 72.3% +/- 0.8% for the SM, and MDP = 57.5% +/- 0.8% for the MLM), so we claim a non-detection of polarization for this GRB. Using the MLM, we placed a 90% confidence upper limit of 46% on the polarization level. GRBs with polarization levels higher than 46% have been reported in the literature, and so this result will be useful in future meta-analyses that consider the statistical distribution of polarization levels across the GRB population.