The Gamma-Ray Burst Polarimeter - POLAR onboard the China’s Spacelab “Tiangong-2”

POLAR is a novel compact space-borne Compton polarimeter conceived for a precise measurement of hard X-ray/Gamma-ray polarization and optimized for the detection of the prompt emission of Gamma-Ray Bursts (GRB) in the energy range 50 - 500 keV. The future detailed measurement of the polarization of GRB will lead to a better understanding of its radiation region geometry and emission mechanisms. Thanks to its remarkable properties on large effective area, field of view (4.14 sr or 1/3 of the visible sky) and obvious modulation factor, POLAR will be able to reach a minimum detectable polarization (1-σ level) of about 3% for several GRB measurements per year. POLAR consists of 25 detector modular units (DMU). Each DMU is composed of 64 low-Z plastic scintillator bars, read out by a flat-panel multi-anode photomultipliers and ASIC front-end electronics. The incoming photons undergo Compton scattering in the bars and produce a modulation pattern. Simulations and hard X-ray synchrotron radiation experiments have shown that the polarization degree and angle can be retrieved from this pattern with the accuracy necessary for pinning down the GRB mechanisms. A full flight model of POLAR has been constructed, in view of a flight on the Chinese spacelab TG-2 expected in 2016. The design of POLAR, Monte-Carlo simulation analysis as well as the performance test results will be all addressed in details in this talk.