Wide-Field X-ray Polarimetry for High Energy Astronomical Transients: First results of the pathfinder CXPD Cubesat Mission
Hong-Bang Liu, Zu-Ke Feng, Huan-Bo Feng, Di-Fan Yi, Li-Rong Xie, Yan-Jun Xie, Zong-Wang Fan, Jin Zhang, Wen-Jin Xie, Xue-Feng Huang, Wei Deng, Fei Xie, Dong Wang, Zi-Li Li, Hui Wang, Ran Chen, Shi-Qiang Zhou, Kai Chen, Jin Li, Qian Liu, Shi Chen, Rui-Ting Ma, Bin-Long Wang, Zhen-Yu Tang, Hang-Zhou Li, Bo Peng, Shu-Lin Liu, Xiang-Ming Sun, Yang-Heng Zheng, En-Wei Liang
TL;DR
The paper demonstrates the CXPD CubeSat as a pathfinder for wide-field soft X-ray polarization measurements aimed at transient sources, validating detector technology, background rejection, and in-orbit operation. It combines two GMPDs with a Topmetal-II pixel readout and an anti-coincidence detector to enable polarization measurements in the 2–10 keV range with a wide 30.2° FOV. Ground calibrations establish energy response and modulation factors up to ~0.612, while in-orbit observations of Sco X-1 and Swift J1727.8-1613 yield MDPs of 16.9% and 10.7% respectively, with no significant polarization detected within uncertainties. The results confirm the feasibility of wide-field soft X-ray polarization measurements and provide critical guidance for the POLAR-2/LPD mission, including design choices and data-processing workflows for future, larger-area units capable of broader sky coverage and polarization reconstruction of transients.
Abstract
The Low Energy Polarization Detector (LPD) is a key component of the next-generation large-scale Gamma-Ray Burst polarimeter, POLAR-2. It is designed for polarization observations of transient sources in the soft X-ray energy range with a wide field of view (FOV). To validate the key technologies required for wide-FOV X-ray polarization measurements, the Cosmic X-ray Polarization Detector (CXPD) CubeSat was developed as a prototype for the LPD. The CXPD is equipped with two Gas Microchannel Plate Pixel Detectors (GMPDs) that measure X-ray polarization via the photoelectric effect, where ejected photoelectrons produce ionization tracks in the gas which are imaged to reconstruct their emission directions. Laboratory calibrations of the modulation factor and energy spectra were successfully performed using linear polarized X-ray sources at 2.98 keV, 4.51 keV, 6.40 keV, and 8.05 keV. Since its launch in June 2023, the CXPD has successfully completed critical in-orbit technology verification. It has also performed polarization observations of two bright X-ray sources Sco X-1 and the transient Swift J1727.8-1613 yielding constraints on their polarization degrees and angles. Notably, this was the first time that an anti-coincidence detector had been implemented in an X-ray polarimeter, enabling in-orbit verification of the charged-particle background rejection algorithm. These results demonstrate the feasibility of wide-field soft X-ray polarization measurements and provide essential guidance for the development of the LPD for the POLAR-2 mission, thereby advancing the frontier of X-ray polarization astronomy.