XMM-Newton observation of the Lockman Hole II. Spectral analysis

Abstract

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. (2001). 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 1.6 x 10^(-15) erg cm^(-2) s^(-1)) 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] > 10^(44) erg s^(-1) and log(N_H) > 10^(22) cm^(-2): which are likely Type-2 QSOs and we thus derive a density of ∼69 objects of this class per square degree.

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