Overview of the POLAR mission

Abstract

The Gamma-ray Bursts Polarimeter POLAR mission is a dedicated experiment to measure and investigate the polarization details of the Gamma-ray Burst (GRB) prompt emissions with so far the highest precision under the close collaboration between Chinese and European scientists for more than ten years. POLAR has been successfully launched onboard the Chinese space laboratory Tiangong-2 on 15th September, 2016. The POLAR detector is optimized for the hard X-ray/Gamma-ray polarization measurement in 50-500 keV energy range with Compton scattering technology. Along with proper and detailed both ground and in-orbit calibrations and performance study, eventually the POLAR systematic effects are well controlled and relatively small after appropriate amendment. After launch, POLAR has detected 55 GRBs jointly with other instruments. Of these, detailed polarization measurements have been published recently for 5 of them. The analysis of the 5 GRBs shows that the average polarization degree is not as high as some GRB models predicted. A new finding of the evolution of the intrapulse polarization angle provides us with a new insight into the GRB physics. The latest results of POLAR aroused a big interest but also raised several critical scientific questions regarding GRB physics. Thus, the POLAR's successor mission which is POLAR-2 is proposed. Besides, POLAR also detected several Solar Flare events and observed and monitored bright pulsars during operation. POLAR is also able to roughly localize the direction for some bright GRBs. The design and implementation of the POLAR experiment, the performance and scientific observation results of POLAR will be addressed.