POLAR: A novel gamma-ray burst polarimeter

The linear polarization in the prompt emission of gamma ray bursts (GRB) remains one of the crucial and not-yet-determined parameters of GRBs. This quantity, intimately related with the GRB emission mechanism, source geometry, and structure of the surrounding magnetic field will provide very valuable information on the origin of GRBs. POLAR is a space borne hard X-ray polarimeter whose design has been optimized to measure the level of linear polarization of gamma-ray bursts 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 = 400 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% (3σ). We present the design and status of POLAR with the newest results acquired from both Monte Carlo simulations and laboratory measurements.