2002A&A...393..425M

We present the results of the X-ray spectral analysis of the first deep X-ray survey with the XMM-Newton observatory during Performance Verification. The X-ray data of the Lockman Hole field and the derived cumulative source counts were reported by Hasinger et al. (2001A&A...365L..45H). We restrict the analysis to the sample of 98 sources with more than 70 net counts (flux limit in the [0.5-7]keV band of 1.6x10^–15erg/cm²/s) of which 61 have redshift identification. We find no correlation between the spectral index Γ and the intrinsic absorption column density N_H and, for both the Type-1 and Type-2 AGN populations, we obtain <Γ≥∼2. The progressive hardening of the mean X-ray source spectrum with decreasing flux is essentially due to an increase in intrinsic absorption. The marked separation between the two AGN populations in several diagnostics diagrams, involving X-ray colour, X-ray flux, optical/near IR colour and optical brightness, is also a consequence of different absorption column densities and enables the classification of optically faint obscured AGN. The Type-2 and obscured AGN have weaker soft X-ray and optical fluxes and redder R-K' colours. They follow the evolutionary tracks of their host galaxies in a color-redshift diagram. About 27% of the subsample with R-K' colour are EROs (R-K'≥5) and most of these 18 X-ray selected EROs contain an obscured AGN as revealed by their high X-ray-to-optical/near IR flux ratios. There are six sources in our sample with L_X[0.5-10]>10⁴⁴erg/s and log(N_H)>10²²cm^–2: which are likely Type-2 QSOs and we thus derive a density of ∼69 objects of this class per square degree.