Surveys with Athena: results from detailed SIXTE simulations

"Formation and early growth of BH' and "Accretion by supermassive BH through cosmic time' are two of the scientific objectives of the Athena mission. To these and other topics (i.e. first galaxy groups, cold and warm obscuration and feedback signatures in AGN at high z), a large fraction (20-25%) of the Athena Mock Observing Plan is devoted, in the form of a multi-tiered (deep-medium-wide) survey with the WFI. We used the flexible SIXTE simulator to study the impact of different instrumental configurations, in terms of WFI FOV, mirror psf, background levels, on the performance in the three layers of the WFI survey. We mainly focus on the scientific objective that drives the survey configuration: the detection of at least 10 AGN at z=6-8 with Log(LX)=43-43.5 erg/s and 10 at z=8.10 with Log(LX)=44-44.5 erg/s. Implications for other scientific objectives involved in the survey are also discussed.