In-flight Performance of the IXPE Telescopes

We present a comprehensive characterization of the on-orbit imaging performance of the three telescopes on board the Imaging X-ray Polarimetry Explorer (IXPE). Each telescope comprises a Wolter-I mirror module assembly and a Gas Pixel Detector focal-plane detector unit. We analyze data from point-like X-ray sources and fit a composite point-spread function (PSF) model that we compare with ground calibrations. We study the dependence of the PSF parameters and of the angular resolution, in terms of half-power diameter (HPD), on the time and source counting rate. We find no significant secular evolution of PSF parameters or HPD over 30 months on orbit, with average HPDs of 26 . ″ 1 ± 0 . ″ 5 (Telescope 1), 32 . ″ 1 ± 0 . ″ 5 (Telescope 2), and 30 . ″ 9 ± 0 . ″ 6 (Telescope 3) and rate trends consistent with zero up to source counting rates of ∼60 cts s −1 in the 2–3 keV energy band for all three telescopes. We set a 99% C.L. upper limit of 4.4% on the optics-induced polarization in the PSF halo and find no measurable degradation of the polarization modulation factor in the wings versus the core due to misreconstructed photoelectron tracks. IXPE’s imaging performance thus is consistent with the ≤30″ observatory requirement with high stability, ensuring robust spatially resolved polarization measurements for the mission’s projected lifetime through 2030.