Re-assessment of the performances of the optical blocking filters for X-IFU observations of massive stars

X-ray bright massive stars have been selected for in-depth studies of their X-ray spectra with the X-IFU. Since OB stars have log(LX/Lbol) ~ -7 and their spectral energy distributions peak in the UV, the flux of optical and UV photons reaching the Athena telescope compared to the flux of X-ray photons can be substantial. Indeed, the X-ray brightest massive stars are also bright optical and UV targets. Optical and UV photons reaching the X-IFU focal plane lead to a degradation of the instrument’s energy resolution. Therefore, in addition to the thermal filters carried by the X-IFU Aperture Cylinder, the filter wheel of the X-IFU should carry at least one optical blocking filter to attenuate the flux of non-X-ray photons. In a previous technical note, we assessed the performances of various filters for observing OB stars up to an mV magnitude of 2 (as currently foreseen in the science plan of SWG 3.2). Since the design of the thermal and optical blocking filters has changed over recent months as part of an exercise to optimize the effective area in the soft X-ray band, it is time to re-consider this issue once again.