Mock catalogs for the extragalactic X-ray sky: simulating AGN surveys
  with Athena and with the AXIS probe
  We present a series of new, publicly available mock catalogs of X-ray
selected active galactic nuclei (AGNs), non-active galaxies, and clusters of
galaxies. They are based on up-to-date observational results on the demographic
of extragalactic X-ray sources and their extrapolations.These mocks reach
fluxes below 1E-20 erg s-1 cm-2 in the 0.5-2 keV band, i.e., more than an order
of magnitude below the predicted limits of future deep fields, and therefore
represent an important tool for simulating extragalactic X-ray surveys with
both current and future telescopes. We use our mocks to perform a set of
end-to-end simulations of X-ray surveys with the forthcoming Athena mission and
with the AXIS probe, a sub-arcsecond resolution X-ray mission concept proposed
to the Astro 2020 Decadal Survey. We find that these proposed, next generation
surveys may transform our knowledge of the deep X-ray Universe. As an example,
in a total observing time of 15\,Ms, AXIS would detect ~225,000 AGNs and
~$50,000 non-active galaxies, reaching a flux limit f_0.5-2~5E-19 erg/s/cm2 in
the 0.5-2 keV band, with an improvement of over an order of magnitude with
respect to surveys with current X-ray facilities. Consequently, 90% of these
sources would be detected for the first time in the X-rays. Furthermore, we
show that deep and wide X-ray surveys with instruments like AXIS and Athena are
expected to detect ~20,000 z>3 AGNs and ~250 sources at redshift z>6, thus
opening a new window of knowledge on the evolution of AGNs over cosmic time and
putting strong constraints on the predictions of theoretical models of black
hole seed accretion in the early universe.
