First Results of POLAR: A dedicated Gamma-Ray Burst Polarimeter

Abstract

This year marks the 50th anniversary of the first detection of a Gamma-Ray Burst. Despite intensive research on these transient events many questions remain regarding their progenitors and the processes responsible for their high energy emission. As the existing models for the prompt emission have largely differing predictions on the polarization parameters, polarimetric measurements are key to the study of Gamma-Ray Bursts. POLAR is a dedicated gamma-ray polarimeter designed to produce the first catalog of polarization measurements of the prompt emission of Gamma-Ray Bursts. The instrument was launched successfully from the Jiquan Satellite Launching Centre in Inner-Mongolia, China, on the 15th of September 2016. POLAR measures the polarization by exploiting the dependency of the azimuthal Compton scattering angle of incoming gamma-rays on the polarization vector of the photons. The azimuthal Compton scattering angle is measured using a finely segmented scintillator array consisting of 1600 plastic bars with a surface area of 6 by 6 mm and a length of 176 mm. They are read out in groups of 64 by 25 flat-panel multi-anode photomultipliers. Due to its large granularity POLAR can measure the photon interaction locations, and therefore the scattering angles, with a high precision resulting in a relatively high modulation factor. The instrument furthermore has a relatively large effective area and a field of view of 1/2 the sky, as a result POLAR is one of the most sensitive detectors currently in orbit in its energy range. A total of 55 Gamma-Ray Bursts as well as the Crab pulsar have been detected by POLAR during the first 6 months and searches for electromagnetic counter parts of gravitational waves as well as high energy neutrino counter parts have been performed during this period. An overview of the POLAR detector will be presented along with the first result of the in-flight performance. Finally the measured Gamma-Ray Bursts will be discussed together with prospects for polarization measurements of these events and the future prospects of the experiment.