In-flight calibration of the <i>Lobster Eye Imager for Astronomy</i>

The Lobster Eye Imager for Astronomy (LEIA), as a pathfinder of the Wide-field X-ray Telescope (WXT) on board the Einstein Probe ( EP ) satellite, is the first lobster-eye-focusing X-ray telescope with a considerably large field of view (FoV) to ever be flown. During the two and a half years of operations, a series of calibration observations were performed to fully characterize its in-orbit performance and calibrate the instrumental properties. In this paper, we present the results of the in-flight calibration campaign of LEIA , focusing on the properties of the point spread function (PSF), source positional accuracy, effective area, energy response, and the instrumental background. The calibration sources used are the Crab Nebula, Scorpius X-1, and the Cassiopeia A supernova remnant. Specifically, it is found that the spatial resolution (represented by the full width at half maximum of the elliptical PSF focal spot in the direction of the long axis) remains almost unchanged compared to the prelaunch values, ranging from 3.6′-9.3′ with a median of 5.9′. The post-calibration source positional accuracy is found to be ∼2′ (at the 90% confidence level). The Crab spectra can be well reproduced by an absorbed power-law model with the best-fit parameters largely in agreement with the literature values, indicating that the inorbit effective area is overall consistent with the model predictions and ground measurements. The effective area exhibits a systematic uncertainty of ≲10% (at the 68% C.L.), and a mild deterioration of ∼15% at the lower-energy end after one year of operation. The Cas A spectral analysis shows that the energy scale and spectral resolution of the detectors are generally consistent with ground values. The instrumental background is found to be largely consistent among the four detectors, with strong modulations by geomagnetic activity and the spectrum qualitatively consistent with our previous simulations. These instrumental performances meet the design requirements well. This work paves the way for the in-orbit calibration of the EP -WXT.