Localization of Gamma-ray Bursts using the Compton polarimeter POLAR

The Gamma-ray Burst polarimeter - POLAR is a dedicated instrument for polarization measurements of prompt emissions of the Gamma-ray Bursts (GRBs). POLAR detected 55 GRBs jointly with other satellites in about 6 months after launch onboard the Chinese Space Laboratory “Tiangong-2 (TG-2)” in September 2016. We simulated GRBs from various directions to get the trigger-counting distribution responses of the 1600 pixels of the 25 detector modules of POLAR. The observed trigger-counting distribution of each GRB was compared with the simulated distributions to find the most likely location of the GRB. We studied the localization capabilities of POLAR using the data of 30 GRBs which have reference locations from other GRB detection instruments, as well as the systematic error on GRB localization of POLAR using a Bayesian method. The results show that POLAR is able to localize GRBs with a systematic error of about 3.8∘. For typical GRB in 10 keV to 1000 keV with a fluence from 10−5erg/cm2 to 10−4erg/cm2, the statistical error of localization is between 10° to 2.8°. Further simulation studies were performed to understand the systematic error on the polarization measurements caused by the location errors.