Testing the X-IFU calibration requirements: An example for quantum efficiency and energy resolution

With its array of 3840 Transition Edge Sensors (TESs) operated at 90 mK, the X-Ray Integral Field Unit (XIFU) on board the ESA L2 mission Athena will provide spatially resolved high-resolution spectroscopy (2.5 eV FWHM up to 7 keV) over the 0.2 to 12 keV bandpass. The in-flight performance of the X-IFU will be strongly affected by the calibration of the instrument. Uncertainties in the knowledge of the overall system, from the filter transmission to the energy scale, may introduce systematic errors in the data, which could potentially compromise science objectives - notably those involving line characterisation e.g. turbulence velocity measurements - if not properly accounted for. Defining and validating calibration requirements is therefore of paramount importance. In this paper, we put forward a simulation tool based on the most up-to-date configurations of the various subsystems (e.g. filters, detector absorbers) which allows us to estimate systematic errors related to uncertainties in the instrumental response. Notably, the effect of uncertainties in the energy resolution and of the instrumental quantum efficiency on X-IFU observations is assessed, by taking as a test case the measurements of the iron K complex in the hot gas surrounding clusters of galaxies. In-flight and ground calibration of the energy resolution and the quantum efficiency is also addressed. We demonstrate that provided an accurate calibration of the instrument, such effects should be low in both cases with respect to statistics during observations.