POLAR: a Gamma-Ray Burst Polarimeter onboard the Chinese Spacelab

POLAR is a novel compact space-borne Compton polarimeter conceived for a precise measurement of hard X-ray polarization and optimized for the detection of the prompt emission of Gamma-Ray Bursts (GRB) in the energy range 50−500 keV. A first detailed measurement of the polarization from astrophysical sources will lead to a better understanding of the source geometry and of the emission mechanisms. Thanks to its large modulation factor, effective area, and field of view (1/3 of the visible sky), POLARwill be able to reach a minimum detectable polarization (1-σ level) of about 3% for several GRB measurements per year. POLAR consists of 1600 low-Z plastic scintillator bars, read out by 25 flat-panel multi-anode photomultipliers. The incoming photons undergo Compton scattering in the bars and produce a modulation pattern; simulations and 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. Details on the qualification test campaign of the POLAR Engineering and Qualification Model are described in detail elsewhere in this conference (see abstract ID:1351). A first full-scale copy of the flight model of POLAR is currently under construction in Geneva based on the positive outcome of the qualification tests, in view of a flight on the Chinese spacelab TG-2 expected in 2014.