Deep X-ray surveys indicate that the cosmic X-ray background (XRB) is largely due to accretion onto supermassive black holes, integrated over cosmic time. In the soft (0.5-2 keV) band 80-90% of the XRB flux has been resolved using ROSAT and recent Chandra surveys (Hasinger et al. 1998a; Mushotzky et al. 2000; Giacconi et al. 2000). In the harder (2-10 keV) band 25-30% of the background have been resolved in ASCA and BeppoSAX surveys (Ueda et al. 1998; Cagnoni et al. 1998; Giommi et al. 2000), and more than 60%, when the recent Chandra surveys are included. Surveys in the very hard (5-10 keV) band have been pioneered using BeppoSAX and resolve about 30% of the XRB (Fiore et al. 1999). Those X-ray surveys with a high degree of completeness in optical spectroscopy find predominantly Active Galactic Nuclei (AGN) as counterparts of the faint X-ray source population (Bower et al. 1996; Schmidt et al. 1998; Zamorani et al. 1999; Akiyama et al. 2000), mainly X-ray and optically unobscured AGN (type-1 Seyferts and QSOs) but also a smaller fraction of obscured AGN (type-2 Seyferts). Spectroscopic identifications of the BeppoSAX and Chandra surveys are still far from complete, however a mixture of obscured and unobscured AGN seems to be the dominant population in these samples, too (Fiore et al. 2000; Barger et al. 2000; Giacconi et al. 2000). The most recent AGN X-ray luminosity function, derived from the ROSAT surveys, shows evidence for luminosity-dependent density evolution and indicates a constant QSO space density at redshifts 2 < z < 4 (Hasinger 1998; Miyaji et al. 2000) unlike optical QSO luminosity functions (Schmidt et al. 1995; Fan et al. 2000).
The X-ray observations are consistent with population synthesis models based on unified AGN schemes (Setti & Woltjer 1989; Madau et al. 1993; Comastri et al. 1995; Gilli et al. 1999), which explain the hard spectrum of the X-ray background by a mixture of absorbed and unabsorbed AGN, folded with the corresponding luminosity function and its cosmological evolution. According to these models most AGN spectra are heavily absorbed and about 80% of the light produced by accretion will be absorbed by gas and dust (Fabian et al. 1998). However, these models are not unique and contain a number of hidden parameters, so that their predictive power remains limited (e.g. Hasinger 2000). In particular they require a substantial contribution of high-luminosity obscured X-ray sources (type-2 QSOs), which so far have not been detected in sufficient quantities (see the discussion in Halpern et al. 1999). The large throughput and the unprecedented hard X-ray sensitivity of the telescopes aboard the recently launched XMM-Newton observatory (hereafter XMM; Jansen et al. 2001) will ultimately yield spectra of the faint X-ray sources and constrain the evolution of their physical properties, in particular the X-ray absorption.
Here we present results of the first deep survey taken with XMM in the Lockman Hole, one of the best studied sky areas at all wavelengths. This paper concentrates on the X-ray data analysis. We show combined images from the EPIC pn-CCD (Strüder et al. 2001) and MOS CCD cameras (Turner et al. 2001) and the derived source counts in different energy bands. With the help of X-ray colour-colour diagrams and the previously identified sources in this field we show that it is possible to obtain a coarse source classification based on XMM data alone.
© ESO 2001