Ground calibration plan for the Athena/X-IFU microcalorimeter spectrometer

The X-ray Integral Field Unit is the X-ray imaging spectrometer on-board one of ESA's next large missions, Athena. Athena is set to investigate the theme of the Hot and Energetic Universe, with a launch planned in the late-2030s. Based on a high sensitivity Transition Edge Sensor (TES) detector array operated at very low temperature (50 mK), X-IFU will provide spatially resolved high resolution spectroscopy of the X-ray sky in the 0.2-12 keV energy band, with an energy resolution goal of 4 eV up to 7 keV [3 eV design goal]. This paper presents the current calibration plan of the X-IFU. It provides the requirements applicable to the X-IFU calibration, describes the overall calibration strategy, and details the procedure and sources needed for the ground calibration of each parameter or characteristics of the X-IFU.