POLAR: a space borne GRB polarimeter

The direction and the level of polarization of high energy photons emitted by astrophysics sources are valuable observables for the understanding of the corresponding emission mechanisms, source geometry and strength of magnetic fields at work. POLAR is a novel compact space-borne detector conceived for a precise measurement of hard X-ray polarization and optimized for the detection of Gamma-Ray Burst (GRB) photons in the energy range 50-500 keV. In POLAR, the GRB photons undergo Compton scattering in a target made out of 1600 plastic scintillator bars. The azimuthal distribution of the scattered photons inside the target provides the information on the GRB polarization. The target is divided into 5 5 units, each one consisting of 8 8 scintillator bars optically coupled with a multi-anode photomultiplier. POLAR, thanks to its large modulation factor (μ100=40%), its large effective area (Aeff = 250 cm2), and its large field of view ( 1/3 of the sky) will be able to determine the degree and angle of polarization of a strong GRB with a minimum detectable polarization of less than 10% (3 σ ). In this communication the present design and status of the POLAR project is presented. Expected results through deep Monte Carlo simulation studies as well as the recent results of laboratory measurements are detailed.