POLAR伽马暴瞬时辐射偏振测量与脉冲星导航试验研究进展

Research progress on polarization measurements of gamma-ray burst prompt emissions and the test of pulsar navigation with POLAR

伽马射线暴(简称伽马暴,gamma-ray burst,GRB)从发现至今已超过半个世纪,其观测样本得到了极大提高,理论研究也取得了瞩目的成果.但对伽马暴的研究还有诸多未解之谜,例如伽马暴的分类和起源、爆发过程中的喷流成分、辐射机制和磁场构型等.其中,对于伽马暴的辐射机制,科学家提出了多种理论模型,不同的模型对于伽马暴爆发期间产生的伽马射线偏振状态的预测不同.因此,采用测量偏振的方法,可以对伽马暴的爆发机制进行研究,并且理解产生伽马射线的极端相对论喷流的结构及其磁场的构型等物理问题.然而,测量伽马暴瞬时辐射过程中的偏振具有极高的挑战性.伽马暴偏振探测仪(POLAR)是一台专门用于高精度测量伽马暴瞬时辐射偏振的空间望远镜,2016年9月15日搭载于我国的空间实验室“天宫二号”成功发射.POLAR在轨共探测到55个确认的伽马暴,首批给出了5个伽马暴的高精度偏振测量结果,最终统计性地给出了14个伽马暴的偏振测量结果.同时,POLAR还观测到了Crab脉冲星信号和太阳X射线耀发事件等.本文简要介绍POLAR,并给出其在轨伽马暴偏振测量结果以及利用观测到的脉冲星信号所开展的导航试验研究进展情况.此外,简要介绍POLAR的后续实验项目POLAR-2,对其未来的科学探测能力进行展望.

It has been more than half a century since the serendipitous discovery of gamma-ray bursts(GRBs,a kind of extreme violent emission of short-duration flashes of gamma-rays occurring randomly over time and in space at cosmological distances),made by the Vela satellites.So far,the observed samples of GRBs have been greatly improved,and the theoretical studies have also made remarkable achievements.However,there are still many unanswered questions about GRBs,such as their classification and origin(s),the composition of the relativistic jet in which the gamma-rays are generated,the radiation mechanism and the magnetic field configuration,etc.Among the questions,a variety of theoretical models have been proposed for interpreting the radiation mechanism of GRBs,and different models predict different polarization properties of the gamma-rays generated during prompt emissions.Accordingly,the polarization measurements can be used to study the mechanism of GRBs and to understand the physical properties such as the structure of the extreme relativistic jet that generates gamma-rays and the configuration of its magnetic field.However,technically it has been quite challenging to measure the polarizations of GRBs during their prompt emissions.Fortunately,some newly developed polarimeters in recent years have overcome major technical difficulties on polarization measurements in space,openning a new window for space hard X-ray/gamma-ray polarimetry.The gamma-ray burst polarimeter POLAR,which was launched onboard the China’s space lab“Tiangong-2”on 15th September 2016,is a space mission dedicated for the polarization measurements of GRB prompt emissions in the 50–500 keV energy range,under the collaboration between Chinese and European scientists.During the about 6 months of space observation,POLAR detected 55 confirmed GRBs jointly with other missions and some possible individual GRBs by itself.The first detailed polarization measurements with high precision for 5 GRBs detected by POLAR have been published in 2019,and finally a catalog of 14 GRBs polarization measurement results in total was published,which is the best GRB polarization measurement results so far thanks to the high sensitivity and large field of view of the instrument,as well as the precisely calibrated systematic errors for polarization measurements.The results show that the detected GRBs are at most modestly polarized.Another new finding of the evolution of the intrapulse polarization angle provides us with a new insight into the GRB physics.POLAR results raised large interests as well as several critical scientific questions regarding GRB physics.Furthermore,the scientific potentials of POLAR have been extended during the flight by optimizing the working parameters which enabled the instrument to detect the Crab pulsar for navigation test studies,as well as several solar flares.The POLAR-2 mission,which is the successor of POLAR,aims to answer some key questions raised by POLAR with the launching date around 2025.In this paper,we first give a brief introduction to POLAR,then present the polarization measurement results of detected GRBs and the progress of the navigation test using the observed pulsar signals.In addition,a brief introduction is given to POLAR-2,and its preliminary scientific capabilities are prospected.