Assessment of the performance of optical blocking filters for X-IFU observations of massive stars

Massive stars that are bright X-ray sources, and hence constitute the best candidates for in-depth studies of their X-ray emission with the X-IFU, are most of the time also bright in the optical and UV domain. Optical and UV photons that reach the focal plane of the X-IFU can lead to a degradation of the instrument’s energy resolution. Since OB stars have log(LX/Lbol) ~ -7, the number of optical and UV photons that enter the Athena telescope compared to the number of X-ray photons can be huge. Therefore, in order to preserve the full scientific capabilities of the X-IFU, it is important to ensure an efficient attenuation of the flux of non-X-ray photons that reaches the X-IFU detector. Some attenuation is achieved with the set of five thermal filters that will be located inside the X-IFU
Aperture Cylinder. Yet, in order to reduce the optical loading to an acceptable level, additional optical blocking filters mounted on a filter wheel are needed. In this technical note, we present simulations of the optical load that will result for observations of OB stars up to an mV magnitude of 2 and considering various combinations of thermal and optical blocking filters.