Gamma-Ray Burst Polarization: Limits from RHESSI Measurements

Using the RHESSI satellite as a Compton polarimeter, a recent study claimed that the prompt emission of GRB 021206 was almost fully linearly polarized. This was challenged by a subsequent reanalysis. We present a novel approach, applying our method to the same data. We identify Compton scattering candidates by carefully filtering events in energy, time, and scattering geometry. Our polarization search is based on time-dependent scattering rates in perpendicular directions, thus optimally excluding systematic errors. We perform simulations to obtain the instrument's polarimetric sensitivity, and these simulations include photon polarization. For GRB 021206, we formally find a linear polarization degree of Π_GRB = 41%, concluding that the data quality is insufficient to constrain the polarization degree in this case. We further applied our analysis to GRB 030519B and again found a null result.