Polarimetry with POLAR

Abstract

The year 2017 marked the 50th anniversary of the detection of the first Gamma-Ray Burst. The 5 decades following the discovery have seen a large interest in these transient phenomena accompanied by both dedicated space and ground-borne detectors to study the emission over a wide energy range. However, many questions on the progenitors of the bursts, the emission processes involved and the emission environments still remain. Some of the most important remaining questions can be solved by probing one of the remaining unstudied parameters of the high energy component of the emission, the polarization. POLAR is a space-borne detector designed specifically to measure the polarization of the prompt emission of GRBs in the 50-500 keV energy range. For this purpose the instrument uses a segmented scintillator array to measure the potential asymmetry in the azimuthal Compton scattering angle induced by the polarized component of the photon flux. The POLAR instrument was launched as part of the second Chinese spacelab, the Tiangong-2 on the 15th of September 2016 from the Jiuquan Satellite Launching Centre in Inner-Mongolia, China. Due to its relatively small pixel size POLAR is able to measure the photon interaction locations, and therefore the scattering angles, with a high precision resulting in a relatively high sensitivity to polarization. The instrument furthermore has a large effective area and a field of view of approximately 1/2 the sky, making POLAR optimal for detecting transient events and making it one of the most sensitive detectors currently in orbit in its energy range. In the first half year of operation POLAR detected a total of 55 Gamma-Ray Bursts about 10 of which were bright enough to allow for detailed polarization studies, thereby forming the start of the first Gamma-Ray Burst polarization catalog. In this paper a brief overview of the previous GRB polarization studies will be presented followed by an overview of the POLAR detector along with the first result of the in-flight performance. The detected Gamma-Ray bursts will be presented and finally prospects for polarization measurements of these events will be discussed.