POLAR: A Gamma-Ray Burst polarimeter in space

The POLAR experiment is a joint European-Chinese project 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 50500 ke V. 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, large effective area, and wide field of view (1/3 of the visible sky), POLAR will be able to reach a minimum detectable polarization (1-σ level) of about 3% for several GRB measurements per year. POLAR is a novel compact space-borne Compton polarimeter consisting of 1600 low-Z plastic scintillator bars, read out by 25 flat-panel multianode photomultipliers. The incoming photons undergo Compton scattering in the bars and produce a modulation pattern; experiments with polarized synchrotron radiation and GEANT4 Monte Carlo simulations have shown that the polarization degree and angle can be retrieved from this pattern with the accuracy necessary for pinning down the GRB mechanisms. The European Space Agency financed (through its PRODEX office) in December 2011 the construction of three copies of the POLAR detectors: two full-scale copies of the flight model are currently under construction in Geneva and will undergo a space qualification campaign; the flight model will be placed onboard the Chinese spacelab TG-2, scheduled for launch in low Earth orbit in 2014.