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G. Hasinger (Astrophysikalisches Institut Potsdam)
After deep ROSAT surveys have resolved 70-80% of the soft (0.5-2 keV) XRB flux, recent Chandra and XMM-Newton surveys have resolved the majority of the XRB also in hard (2-10 keV and 5-10 keV) bands. 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, mainly X-ray and optically unobscured AGN (Sy-1 and QSO) but also a smaller fraction of obscured AGN (Sy-2). Spectroscopic identifications of the 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. The X-ray observations are consistent with population synthesis models based on unified AGN schemes, 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 clouds. However, these models are far from unique and their predictive power remains limited. 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. I will discuss the preliminary results of the recent Chandra and XMM surveys in the light of these models.