AGN and QSOs in the eROSITA All-Sky Survey

I. Statistical properties

¹ Max-Planck-Institut für Astrophysik (MPA), Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
e-mail: alex@mpa-garching.mpg.de
² Space Research Institute (IKI), Russian Academy of Sciences, Profsoyuznaya ul. 84/32, 117997 Moscow, Russia
³ Moscow Institute of Physics and Technology, Institutsky per. 9, 141700 Dolgoprudny, Russia
⁴ Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse, Postfach 1312, 85741 Garching, Germany
⁵ Dipartimento di Fisica e Astronomia, Universitá di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁶ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 10 December 2012
Accepted: 15 August 2013

Abstract

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 active galactic nuclei (AGN) and quasars/quasi-stellar objects (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 about 1 × 10^-14 ergs^-1 cm^-2 will be achieved in the 0.5−2.0 keV band. With this sensitivity, eROSITA is expected to detect about 3 million AGN on the extragalactic sky (|b| > 10°). The median redshift of the eRASS AGN will be z ≈ 1 with approximately 40% of the objects in the z = 1−2 redshift range. About 10⁴−10⁵ AGN are predicted beyond redshift z = 3 and about 2000−30 000 AGN beyond redshift z = 4, the exact numbers depend on the poorly known behavior of the AGN XLF in the high-redshift and high-luminosity regimes. Of the detected AGN, the brightest 10% will be detected with more than ≈38 counts per point-spread-function (half-energy width), while the faintest 10% will have fewer than ≈9 counts. The optical counterparts of approximately 95% of the AGN will be brighter than I_AB = 22.5 mag. 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 0.5−2.0 keV sensitivity of ≈2 × 10^-14 to ≈2 × 10^-13 ergs^-1 cm^-2, respectively.