The four-year X-ray all-sky survey (eRASS) of the eROSITA telescope aboard the Spektrum-Roentgen-Gamma satellite will detect ~3 million active galactic nuclei (AGN) with a median redshift of z~1 and a typical luminosity of L_(0.5-2.0 keV) ~ 10^(44) e
rg/s. We show that this unprecedented AGN sample, complemented with redshift information, will supply us with outstanding opportunities for large-scale structure research. For the first time, detailed redshift- and luminosity-resolved studies of the bias factor for X-ray selected AGN will become possible. The eRASS AGN sample will not only improve the redshift- and luminosity-resolution of these studies, but will also expand their luminosity range beyond L_(0.5-2.0 keV) ~ 10^(44) erg/s, thus enabling a direct comparison of the clustering properties of luminous X-ray AGN and optical quasars. These studies will dramatically improve our understanding of the AGN environment, triggering mechanisms, the growth of supermassive black holes and their co-evolution with dark matter halos. The eRASS AGN sample will become a powerful cosmological probe. It will enable detecting baryonic acoustic oscillations (BAOs) for the first time with X-ray selected AGN. With the data from the entire extragalactic sky, BAO will be detected at a >~10sigma confidence level in the full redshift range and with ~8sigma confidence in the 0.8 < z < 2.0 range, which is currently not covered by any existing BAO surveys. To exploit the full potential of the eRASS AGN sample, photometric and spectroscopic surveys of large areas and a sufficient depth will be needed.
Context. The main element of the observing program of the Spectrum-Roentgen-Gamma orbital observatory is a four-year all-sky survey, in the course of which the entire sky will be scanned eight times. Aims. We analyze the statistical properties of A
GN and QSOs that are expected to be detected in the course of the eROSITA all-sky survey (eRASS). Methods. According to the currently planned survey strategy and based on the parameters of the Galactic and extragalactic X-ray background as well as on the results of the recent calculations of the eROSITA instrumental background, we computed a sensitivity map of the eRASS. Using the best available redshift-dependent AGN X-ray luminosity function (XLF), we computed various characteristics of the eRASS AGN sample, such as their luminosity- and redshift distributions, and the brightness distributions of their optical counterparts. Results. After four years of the survey, a sky-average sensitivity of ~1x10^(-14) erg/s/cm^2 will be achieved in the 0.5-2.0keV band. With this sensitivity, eROSITA is expected to detect ~3 million AGN on the extragalactic sky (|b|>10deg). The median redshift of the eRASS AGN will be z~1 with ~40% of the objects in the z=1-2 redshift range. About 10^4 - 10^5 AGN are predicted beyond redshift z=3 and about 2 000 - 30 000 AGN beyond redshift z=4, the exact numbers depend on the poorly known behavior of the AGN XLF in the high-redshift and luminosity regimes. Of the detected AGN, the brightest 10% will be detected with more than ~38 counts per PSF HEW, while the faintest 10% will have fewer than ~9 counts. The optical counterparts of ~95% of the AGN will be brighter than I_(AB)=22.5mag. The planned scanning strategy will allow one to search for transient events on a timescale of half a year and a few hours with a 0.5-2.0keV sensitivity of ~2x10^(-14) to ~2x10^(-13) erg/s/cm^2, respectively.
