Calibration of the MXT camera before launch of the SVOM mission and prediction of its spectral performance at the end of the mission

SVOM (Space based Variable Object Monitor) is a Chinese-French mission dedicated to the study of the most luminous explosions in the Universe: Gamma-Ray Bursts. This observatory for time-domain astrophysics is due for launch on June 24th 2024. Among the four space borne instruments is the Micro-channel X-ray Telescope (MXT). The MXT is a focusing X-ray telescope, based on "lobster-eye" optics, whose main goal is to improve the localization of transient sources, as well as to measure their timing and spectral properties. The MXT camera is implementing a 256 × 256 pixels pnCCD detector, sensitive in the 0.2-10 keV energy range. The spectral performance of the MXT instrument was measured in 2021 during the calibration campaign at the MPE PANTER X-ray facility and the End-to-End testing during the vacuum and thermal tests of the full satellite in the SECM Shanghai integration facility. SVOM is in a low-Earth orbit crossing the South Atlantic Anomaly, and the MXT will thus be submitted to irradiation, in particular from protons, that will cause radiation damage. To anticipate the evolution of the MXT performances over its three years mission lifetime, a spare flight model of the MXT detector plane has been irradiated with 50 MeV protons at the Arronax cyclotron facility, and then installed and characterized at the X-ray Metrology beamline of the SOLEIL Synchrotron in June 2023. This paper presents the results of calibration campaigns to infer the performances of the MXT instrument over the lifetime of the SVOM mission.