Studies of the geometric effect for gaseous X-ray polarimetry

Abstract The geometric effect is an intrinsic response of gaseous X-ray polarimetry, which exhibits some sharp peaks with a fixed period angle on modulation curves, especially in observations of unpolarized sources, resulting in spurious modulation and a deviation of polarization detection. We construct a simulation framework to study the origin of the geometric effect and evaluate the impact on the performance of the polarimetry. The modulation curves from the simulation are consistent with measurements while focusing on the geometric effect. The studies show that the geometric effect is from the photoelectrons generated close to multiplier stages, a more significant geometric effect with a deeper depth of the photoelectric impact point. The effect was characterized by a diffusion coefficient related to photoelectric drift height to quantify the geometric effect. From the simulation results, choosing smaller detector units could suppress the effect, and the size of the detector unit is 1.25 times larger than the diffusion coefficient related to an observable geometric effect. In addition, the relative contribution to the effect is approximately 65% for the GEM hole pitch and 35% for the readout pitch. This study provides a reference for detection unit optimization and the threshold setting of fired pixels in observations.