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.

arXiv commentAccepted to ApJ. 9+3 pages. Revision corrected typos in Eq. 7 and B2

References in corpus (7)

• An efficient photoelectric X-ray Polarimeter for the study of Black Holes and Neutron Stars
• ixpeobssim: a Simulation and Analysis Framework for the Imaging X-ray Polarimetry Explorer
• Analyzing the Data from X-ray Polarimeters with Stokes Parameters
• Deep Ensemble Analysis for Imaging X-ray Polarimetry
• Towards Optimal Signal Extraction for Imaging X-ray Polarimetry
• Polarisation Leakage due to Errors in Track Reconstruction in Gas Pixel Detectors
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