Defocussing of the Athena optics is now a requirement and can be considered in evaluating the count rate performance of the X-IFU. Defocussing spreads the source counts over a large number of pixels (1000). Each pixel can sustain a count rate of the order of 40-50 cps. A Crab like source would produce about 90 000 cps over the full TES array. Therefore it is obvious that with defocussing, the X-IFU is capable of observing bright point sources.
High throughput for mid and high resolution events (spectral resolution less than 10 eV) is certainly required around the Iron like between 5 and 8 keV. The use of Be filters would remove a large fraction of the soft photons and therefore reduce the overall count rate of point sources while preserving the interesting events above 5 keV. The X-IFU can achieve throughputs as high as 70% with maximum defocussing and the use of Be filter. The above number will reduce (by a few 10% at 1 Crab) if events affected by cross talk are removed.
The figures shown above are courtesy of Philippe Peille (December 2016), on behalf of the end-to-end simulator team.