In-flight calibration of the Wide-field X-ray Telescope on board the Einstein Probe

By utilizing novel lobster-eye optics, the Wide-field X-ray Telescope (WXT) on board the Probe satellite achieves an unprecedented combination of a large instantaneous field of view (FoV) and high sensitivity for monitoring the dynamic X-ray sky. In this paper, we present the in-orbit calibration results of the WXT during its first two and a half years of operations. By conducting observations of standard celestial sources---i.e., the Crab Nebula, Scorpius X-1, and Cassiopeia A---we systematically characterized key instrumental properties. Our analysis demonstrates that the in-orbit performance of the WXT agrees with prelaunch ground calibrations. The spatial resolution, denoted by the full width at half maximum of the focal spot, typically ranges from 3' to 6' across ∼90% of the FoV, with a median of ∼ 4.3'. The post-calibration source positioning accuracy achieves 1.3' (at the 90% confidence level). The in-orbit effective area is consistent with model predictions and ground measurements, exhibiting an overall systematic uncertainty of łesssim 10% (90% C.L.) in the 0.5--4,keV band. While the vast majority of the detectors remain highly stable, a noticeable long-term degradation at the low-energy end (sim30%--40%, 0.4--0.6,keV) is observed in a few specific modules. Furthermore, spectral evaluations using Cas A confirm the stability of the energy scale and spectral resolution of the focal-plane complementary metal-oxide semiconductor detectors. All derived calibration products have been incorporated into the WXT calibration database (CALDB). These results comprehensively verify the instrumental capabilities of the WXT, providing a solid foundation for the reliable analysis of scientific observations. Einstein