POLAR: A Space-borne X-Ray Polarimeter for Transient Sources

Abstract. POLAR is a novel compact Compton X-ray po-larimeter designed to measure the linear polarization of the prompt emission of Gamma Ray Bursts (GRB) and other strong transient sources such as soft gamma repeaters and solar flares in the energy range 50−500 keV. A detailed mea-surement of the polarization from astrophysical sources will lead to a better understanding of the source geometry and emission mechanisms. POLAR is expected to observe every year several GRBs with a minimum detectable polarization smaller than 10%, thanks to its large modulation factor, ef-fective area, and field of view. POLAR consists of 1600 low-Z plastic scintillator bars, divided in 25 independent modular units, each read out by one flat-panel multi-anode photomul-tiplier. The design of POLAR is reviewed, and results of tests of one modular unit of the engineering and qualification model (EQM) of POLAR with synchrotron radiation are pre-sented. After construction and testing of the full EQM, we will start building the flight model in 2011, in view of the launch foreseen in 2013.