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The Gamma-ray Burst polarimeter - POLAR is a dedicated instrument for polarization measurements of prompt emissions of the Gamma-ray Bursts (GRBs). POLAR detected 55 GRBs jointly with other satellites in about 6 months after launch onboard the Chinese Space Laboratory “Tiangong-2 (TG-2)” in September 2016. We simulated GRBs from various directions to get the trigger-counting distribution responses of the 1600 pixels of the 25 detector modules of POLAR. The observed trigger-counting distribution of each GRB was compared with the simulated distributions to find the most likely location of the GRB. We studied the localization capabilities of POLAR using the data of 30 GRBs which have reference locations from other GRB detection instruments, as well as the systematic error on GRB localization of POLAR using a Bayesian method. The results show that POLAR is able to localize GRBs with a systematic error of about 3 . 8 ∘ . For typical GRB in 10 keV to 1000 keV with a fluence from 1 0 − 5 erg/cm 2 to 1 0 − 4 erg/cm 2 , the statistical error of localization is between 10° to 2.8°. Further simulation studies were performed to understand the systematic error on the polarization measurements caused by the location errors.

Gamma-ray bursts are the strongest explosions in the Universe since the Big Bang, believed to be produced either in forming black holes at the end of massive star evolution or merging of compact objects. Spectral and timing properties of gamma-ray bursts suggest that the observed bright gamma-rays are produced in the most relativistic jets in the Universe; however, the physical properties, especially the structure and magnetic topologies in the jets are still not well known, despite several decades of studies. It is widely believed that precise measurements of the polarization properties of gamma-ray bursts should provide crucial information on the highly relativistic jets. As a result there have been many reports of gamma-ray burst polarization measurements with diverse results, see, however many such measurements suffered from substantial uncertainties, mostly systematic. After the first successful measurements by the GAP and COSI instruments, here we report a statistically meaningful sample of precise polarization measurements, obtained with the dedicated gamma-ray burst polarimeter, POLAR onboard China's Tiangong-2 spacelab. Our results suggest that the gamma-ray emission is at most polarized at a level lower than some popular models have predicted; although our results also show intrapulse evolution of the polarization angle. This indicates that the low polarization degrees could be due to an evolving polarization angle during a gamma-ray burst., 34 pages, 15 figures (16 pages and 3 figures without supplementary information). Accepted for publication in Nature Astronomy, http://dx.doi.org/10.1038/s41550-018-0664-0

POLAR is a compact space-borne detector initially designed to measure the polarization of hard X-rays emitted from Gamma-Ray Bursts in the energy range 50-500keV. This instrument was launched successfully onboard the Chinese space laboratory Tiangong-2 (TG-2) on 2016 September 15. After being switched on a few days later, tens of gigabytes of raw detection data were produced in-orbit by POLAR and transferred to the ground every day. Before the launch date, a full pipeline and related software were designed and developed for the purpose of quickly pre-processing all the raw data from POLAR, which include both science data and engineering data, then to generate the high level scientific data products that are suitable for later science analysis. This pipeline has been successfully applied for use by the POLAR Science Data Center in the Institute of High Energy Physics (IHEP) after POLAR was launched and switched on. A detailed introduction to the pipeline and some of the core relevant algorithms are presented in this paper., 15 pages, 5 figures, 3 tables; Preprint accepted by RAA

14 pages, 8 figures. The paper has been accepted by Research in Astronomy and Astrophysics (RAA), Rapid response and short time latency are very important for Time Domain Astronomy, such as the observations of Gamma-ray Bursts (GRBs) and electromagnetic (EM) counterparts of gravitational waves (GWs). Based on near real-time Fermi/GBM data, we developed a low-latency pipeline to automatically calculate the temporal and spectral properties of GRBs. With this pipeline, some important parameters can be obtained, such as T and fluence, within ∼ 20 min after the GRB trigger. For ∼ 90% of GRBs, T and fluence are consistent with the GBM catalog results within 2σ errors. This pipeline has been used by the Gamma-ray Bursts Polarimeter (POLAR) and the Insight Hard X-ray Modulation Telescope (Insight-HXMT) to follow up the bursts of interest. For GRB 170817A, the first EM counterpart of GW events detected by Fermi/GBM and INTEGRAL/SPI-ACS, the pipeline gave T and spectral information 21 min after the GBM trigger, providing important information for POLAR and Insight-HXMT observations. © 2018 National Astronomical Observatories, CAS and IOP Publishing Ltd.., We thank the anonymous reviewer for helpful comments and suggestions. This work is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB23040400), the National Natural Science Foundation of China (Grant Nos. 11403026, 11503028 and 11673023) and the National Basic Research Program of China (973 program, Grant No. 2014CB845800). BBZ acknowledges support from the National Thousand Young Talents programof China. We gratefully acknowledge support from the collaboration team of POLAR, a project funded by China National Space Administration (CNSA), the Chinese Academy of Sciences (CAS) and the University of Geneva (UNIGE). The authors would also like to thank Jing Jin, Jinlu Qu, Xiaobo Li, Youli Tuo, Zijian Li, Mingyu Ge, Jinyuan Liao, Guangcheng Xiao, Yue Huang and Chengkui Li for their suggestions on data analysis and the revision of this paper.

POLAR is a compact space-borne detector designed to perform reliable measurements of the polarization for transient sources like Gamma-Ray Bursts in the energy range 50-500keV. The instrument works based on the Compton Scattering principle with the plastic scintillators as the main detection material along with the multi-anode photomultiplier tube. POLAR has been launched successfully onboard the Chinese space laboratory TG-2 on 15th September, 2016. In order to reliably reconstruct the polarization information a highly detailed understanding of the instrument is required for both data analysis and Monte Carlo studies. For this purpose a full study of the in-orbit performance was performed in order to obtain the instrument calibration parameters such as noise, pedestal, gain nonlinearity of the electronics, threshold, crosstalk and gain, as well as the effect of temperature on the above parameters. Furthermore the relationship between gain and high voltage of the multi-anode photomultiplier tube has been studied and the errors on all measurement values are presented. Finally the typical systematic error on polarization measurements of Gamma-Ray Bursts due to the measurement error of the calibration parameters are estimated using Monte Carlo simulations., Comment: 43 pages, 30 figures, 1 table; Preprint accepted by NIMA

POLAR is a dedicated Compton scattering polarimeter to detect Gamma-Ray Bursts (GRBs) in the energy range of about 50 to 500 keV, which was launched on-board the Chinese space laboratory Tiangong-2 (TG2) on Sep 15, 2016. Since then, POLAR has detected 55 GRBs jointly with other telescopes as of Apr 1, 2017. Meanwhile, more GRBs detected by POLAR only will be revealed as the burst search and identification program gets more sophisticated to deal with the complicated background. After subtracting the non-observation period due to varies reasons in the commissioning phase, the effective observation time is about 3180 hours, thus the GRB detection rate of POLAR is estimated to be about 150 GRBs/year, demonstrating that POLAR is one of the most sensitive GRB detectors in operation. Thanks to the high sensitivity, large FOV (about 1/2 of the all-sky) and unique capability of polarization measurement, POLAR is a very important detector in the search for high-energy electromagnetic counterpart for Gravitational Wave Bursts (GWB), High Energy Neutrinos (HEN) and Fast Radio Bursts (FRB). In this paper, we summarized the GRB observation by POLAR, including GRB search and identification, joint observation and Gamma-ray Coordinates Network (GCN) alerts, data analysis, GRB catalogues and so on.

Gamma-ray polarimetry is a new powerful tool to study the processes responsible for the emission from astrophysical sources and the environments in which this emission takes place. Few successful polarimetric measurements have however been performed thus far in the gamma-ray energy band due to the difficulties involved. POLAR is a dedicated polarimeter designed to perform high precision measurements of the polarization of the emission from gamma-ray burst in the 50-500 keV energy range. This new polarimeter is expected to detect approximately 50 gamma-ray bursts per year while performing high precision polarization measurements on approximately 10 bursts per year. The instrument was launched into lower earth orbit as part of the second Chinese space lab, the Tiangong-2, on September 15th 2016 and has been taking data successfully since being switched on one week after. The instrument uses a segmented scintillator array consisting of 1600 plastic scintillator bars, read out by 25 flat-panel multi-anode photomultipliers, to measure the Compton scattering angles of incoming photons. The small segmentation and relatively large uniform effective area allow the instrument to measure the polarization of a large number of transient events, such as gamma-ray bursts, with an unprecedented precision during its two year life-time. The final flight model underwent detailed calibration prior to launch as well as intensive space qualification tests, a summary of which will be presented in this paper. The instrument design will be discussed first followed by an overview of the on-ground tests, finally the in-orbit behavior as measured during the first weeks of the mission will be presented.