Wide-field X-ray polarimetry for high energy astronomical transients: First results of the pathfinder CXPD CubeSat mission

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, 4.51, 6.40, 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.