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) onboard the Einstein Probe (EP) 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--including 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 well.

The spatial resolution, denoted by the full width at half maximum (FWHM) of the focal spot, typically ranges from $3'$ to $6'$ across $\sim$90% of the FoV, with a median of $\sim 4.3'$.

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 $\lesssim 10\%$ (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 (CMOS) 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.