Polarization Reconstruction Study of Wide Field-of-view Photoelectric Polarimeter for POLAR-2/LPD

Abstract The Low-Energy X-ray Polarization Detector (LPD) is one of the payloads in the POLAR-2 experiment, which is planned as an external payload for deployment on the China Space Station in 2026. The LPD is specifically designed to observe the polarization of gamma-ray burst prompt emissions in the energy range of 2–10 keV, with a preliminary design featuring a wide field-of-view (FOV) of ±45°. This observation is achieved using an array of X-ray photoelectric polarimeters. Because of the wide FOV design of the detector, nearly all photons impinge on the detector off-axis. This paper investigates the polarization reconstruction of a photoelectric polarimeter for off-axis photons. It was found that the reconstruction of the emission azimuthal angles of photoelectron tracks is the main factor affecting the shape of the modulation function for off-axis radiation. Therefore, a modulation function correction method based on Monte Carlo (MC) simulation is proposed. This MC simulation-based method is applied to both simulated and experimental data to correct the modulation function for off-axis radiation. The reconstruction results demonstrate that this method can accurately determine the polarization degree and polarization angle of photons under different conditions. While we tested up to 45° off-axis angles due to our ±45° FOV, the method is theoretically applicable to off-axis angles up to ∼90°.