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

Abstract

Abstract. POLAR is a novel compact Compton X-ray polarimeter 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 measurement 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, effective 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 photomultiplier. 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 presented. 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.

Afterglow Polarization from Off-axis Gamma-Ray Burst Jets

“…One of the most significant obstacles has been the scarcity of polarization data for GRBs due to the unfortunately small number of orbital polarimeters and the typical difficulties in observing these luminous events. Despite that, advances have been made in the area, and thanks to efforts like the POLAR project (Orsi 2011), it is expected that in future years we should have an abundance of data for the test of different models.…”

Development of a hard x-ray focal plane compton polarimeter: a compact polarimetric configuration with scintillators and Si photomultipliers

“…Though photoelectric polarimeters are expected to provide sensitive polarization measurements, these instruments are effective primarily in soft X-rays where radiation from the source is expected to be less polarized because of the dominance of thermal radiation. Consequently, many groups across the globe are now involved in developing Compton polarimeters effective in hard X-ray regime where the expected polarization is above the typical sensitivity level of the instruments [56,31,29,8,40]. For the same reason, there have been several attempts during the last decade in retrieving polarization information from instruments, not specifically designed for polarimetry but could be sensitive to it [35,14,33,27,19,17,38,21,20,36,37,32,12,50].…”