The Gamma-ray Burst Polarimeter – POLAR is a novel compact space-borne detector conceived for precise measurement of hard X-ray polarization and optimized for the detection of prompt emissions of Gamma-Ray Burst (GRB) photons in the energy range 50-500 keV. GRBs are sudden flashes of gamma-rays that appear randomly in the sky and outshine for typically about 10 seconds and can last from dozens of milliseconds to one thousand seconds in all other gamma-ray sources. They are produced at cosmological distances, and are considered as the brightest events in the universe since the Big Bang. After the first discovery of GRB in 1967, many instruments have performed extensive measurements and studies of this attractive phenomenon, but GRBs’ radiation mechanism and progenitor are still indistinct. Several theories have been elaborated to explain their origin: The fireball, the electromagnetic and the cannonball models are at present the most commonly accepted. All of them relate the emission of the GRB to the creation of a black hole, differing in the physical processes involved in the gamma-ray generation, and also in the level of linear polarization of the prompt emissions. The polarization level and direction of high-energy photons emitted by astrophysics sources such as GRBs are therefore very good observable candidates for unveiling the mystery of emission mechanisms, source geometry and configuration of magnetic fields of the sources during the prompt emission phase.
POLAR consists of two standalone instruments which are polarimeter detector (OBOX) and electric cabinet (IBOX) respectively. OBOX is composed of 25 independent detector modular units (DMUs), central trigger (CT), high voltage power supply (HVPS) and low voltage power supply (LVPS), as well as mechanical supporting structures, etc. Each DMU consists of 64 plastic scintillator (PS) bars (the scintillator material is EJ-248M) in an 8× 8 array, multi-anode photomultipliers H8500 (with 8× 8 pixels) and ASIC front-end electronics (FEE), etc. More details regarding the design of POLAR can be found in N. Produit et al. 2018.
The POLAR experiment is an international collaboration project between China and Europe, implemented within the general collaborative framework between the China Manned Space Agency (CMSA) and the European Space Agency (ESA). The main scientific goal of POLAR is to measure the polarization of the prompt emissions of Gamma-Ray Bursts (GRBs) with high precision. POLAR has been launched successfully aboard the Chinese Space Laboratory “Tiangong-2 (TG-2)” on 15th September, 2016.
During its operation in orbit, POLAR performed well and was accurately calibrated in-orbit. In about six-months, a total of 55 GRBs were detected and confirmed (S. Xiong et al. 2017). The first detailed polarization measurement analysis results for five GRBs have been published online on 14th January in 2019 by Nature Astronomy. The full paper can be retrieved with this link: Shuang-Nan Zhang et al. 2019. This is so far the largest high-precision sample of polarization measurement of GRB prompt emissions. The joint polarization analysis for the five GRBs shows that the average polarization degree (~10%) 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 microcosmic world of the GRB physics. These new observations suggest that the rapid polarization angle change could be dominated by the internal evolution of the relativistic jets in which the intense Gamma-rays are produced.