低系统误差、高灵敏度、符合空间要求的X射线偏振仪研究

X-ray polarimetry will be an important window for future high energy astrophysics. It allows us to constrain the emission mechanism and the orientation of magnetic fields of high energy astrophysical objects. In 2015, the European Space Agency (ESA) selected the X-ray Imaging Polarimetry Explorer (XIPE) project for phase A study. We are one of the proposers, a member of the core team and instrument team, and are involved in the development of the key instrument on XIPE, the focal plane X-ray polarimeter. In the meantime, several future high energy astronomical mission concepts in China have planned to employ X-ray polarimetry as one of the key observational means. We have developed the technique for X-ray polarimetry based on the photoelectric effect. The two most important parameters were measured to be better than those obtained by European and US collaborators. Upon these demands, we will further improve and optimize the design of the X-ray polarimeter and fully calibrate and reduce its systematic errors, try to improve the sensitivity by using a new algorithm for photoelectron track reconstruction, and prove its feasibility in space with simulated space environments. Eventually, we will study how to make X-ray polarimetric observations with a CubeSat, including the scientific justification, instrument design, construction, and test, and will deliver a model ready for flight. The results from these studies will support the down-selection program for XIPE and provide ready techniques for future space astronomy in China.

X射线偏振测量将是高能天体物理未来发展的重要窗口，可以有效限制天体的辐射机制和磁场方位。2015年，欧空局批准了XIPE项目进入phase A研究，我们是项目的联合建议人、核心组和仪器组成员，负责项目最关键的仪器即焦平面X射线偏振仪的部分研究任务。同时，我国未来若干个高能天文观测计划也将X射线偏振列为关键观测手段之一。之前，我们在基于光电效应的偏振测量技术上取得一些成果，最重要的两项技术指标实测优于欧美同行的结果。基于这些需求，本项目将进行进一步改进和优化X射线偏振仪的设计，全面标定并降低系统误差，通过改善电子径迹的重建算法来提高灵敏度，利用空间环境试验来证明仪器的空间应用可行性。最终，将研究如何利用立方星进行空间观测，完成科学论证、仪器设计、硬件建设和调试，达到可发射状态。本项目的研究成果一方面将为XIPE的进一步遴选提供支持，另一方面将直接为我国未来空间天文提供可用的探测技术。

本课题计划基于实验室已有的X射线偏振探测器，进行径迹算法改进、探测器标定、飞行样机研制，并择机发射开展空间天文观测，已经完成了所有既定目标。我们发展了基于图论的新的径迹重建算法，完成了探测器系统误差即残余调制的标定，研制完成了符合立方星接口的空间飞行载荷，通过了一系列空间环境测试，命名为极光计划。极光计划于2019年10月发射上天，目前已经在轨工作超过3年，通过长期观测，发现了蟹状星云脉冲星在自旋突变后偏振的可能变化，在天蝎座X-1中测到了显著的偏振信号，对这些天体的磁场和几何做出了有效限制。极光还对在轨本底进行了建模，找到了有效的粒子甄别方法。