Polarization Leakage and the IXPE PSF

Abstract: By measuring photoelectron tracks, the gas pixel detectors of the Imaging X-ray Polarimetry Explorer satellite provide estimates of the photon detection location and its electric vector position angle (EVPA). However, imperfections in reconstructing event positions blur the image and EVPA-position correlations result in artificial polarized halos around bright sources. We introduce a new model describing this "polarization leakage" and use it to recover the on-orbit telescope point-spread functions, useful for faint source detection and image reconstruction. These point spread functions are more accurate than previous approximations or ground-calibrated products ($\Delta \chi^2\approx 3\times 10^{4}$ and $4 \times 10^4$ respectively for a bright $10^6$-count source). We also define an algorithm for polarization leakage correction substantially more accurate than existing prescriptions ($\Delta \chi^2\approx 1\times 10^{3}$). These corrections depend on the reconstruction method, and we supply prescriptions for the mission-standard "Moments" methods as well as for "Neural Net" event reconstruction. Finally, we present a method to isolate leakage contributions to polarization observations of extended sources and show that an accurate PSF allows the extraction of sub-PSF-scale polarization patterns.