8 Conclusions

We have discussed the X-ray spectral properties of a sample of 98 sources found in the 100 ksec XMM-Newton observation of the Lockman Hole, using data from the EPIC-pn detector. The large throughput and the unprecedented sensitivity at high energies of the X-ray telescope and detectors allow us, for the first time, to measure separetely the intrinsic absorption and the slope of the power law emission spectrum for the faint source population. We have derived the spectral index ($\Gamma$) and the column density ($N_{\rm H}$) for sources with more than 70 counts in the [0.5-7] keV band. We find that the value of $\Gamma$ is independent of the absorption level with $<\Gamma>~ \approx 2$. Thus, we infer that the progressive hardening of the X-ray spectra of faint sources observed in Chandra deep fields (Giacconi et al. 2001; Tozzi et al. 2001; Brandt et al. 2001) is mainly due to the increasing level of intrinsic absorption rather than intrinsically flat spectra.

We confirm that the $R-K^\prime$ colours of X-ray counterparts get redder towards fainter R magnitudes. Such a trend is not present between $R-K^\prime$ and the $K^\prime$ magnitude; this is likely due to a combination of a less pronounced absorption effect in this band, a different K-correction for AGN-type spectra (small) and star-like galaxy spectra (large), as well as an increased contribution of the host galaxy light in the $K^\prime$ band relative to that of the AGN.

Comparing the $R-K^\prime$ colours of the X-ray sources with evolutionary tracks of various galaxy-types as a function of redshift, we find that Type-2 AGN have colours dominated by the host galaxy and are also significantly absorbed ( $\log N_{\rm H}>21.5$). On the other hand, for Type-1 AGN, the large majority of which are unabsorbed, the nuclear component is significantly contributing to their optical colours. In addition, there is a strong correlation between the $R-K^\prime$ colour and the amount of intrinsic X-ray absorption.

We have also defined an X-ray selected sample of 18 EROs ( $R-K^\prime \geq 5$) and found that it mainly comprises X-ray absorbed objects with a strong correlation between colour and intrinsic column density.

We have derived the unabsorbed rest-frame luminosities of the sources with strong intrinsic absorption. There are six absorbed, bright X-ray objects in our sample with $L_{\rm X}[0.5{-}10]>10^{44}$ erg s^-1and $\log(N_{\rm H})>10^{22}$ cm^-2: one is an optically classified Type-1 QSO (source $\char93 96$ see Sect. 7.4), two are Type-2 AGN and the remaining three have a photometric redshift and due to their X-ray absorption and optical/near-IR colours likely Type-2 AGN. Four of them are also EROs ( $R-K^\prime \geq 5$). These are likely to be Type-2 QSO candidates and we derive a density of $\sim$69 objects of this class per square degree at a flux limit in the [0.5-7] keV band of $1.6 \times 10^{-15}$ erg cm^-2 s^-1.

Our analysis of the unidentified sources (mostly newly detected XMM sources) shows that the majority of these sources have absorbed X-ray spectra and are consequently located in the harder part of the diagnostic X-ray colour-colour diagrams. They are also optically fainter ($\sim$80% of them have R>24) and their optical-to-near-IR colours are redder ($\sim$90% have ${R-K}^\prime \geq 4$) than already identified sources. Their X-ray-to-optical flux ratios are $\log (\frac{f_X[2-10]}{f_R}) >1$. From these properties, we argue that the majority of these sources are Type-2 AGN. This is confirmed by our on-going optical spectroscopic survey which is showing that the bulk of these sources is at z<1.

Two X-ray bright optically "normal'' galaxies are present in our sample. Their X-ray spectra are clearly absorbed suggesting the presence of an obscured AGN. We expect this class of objects to increase from the optical identification of the newly detected XMM-Newton sources.

Acknowledgements

We thank Andrea Comastri, Roberto Gilli, Giorgio Matt and Paolo Tozzi for useful comments and discussions. We thank the referee, X. Barcons, for helpful comments that improved the manuscript. RDC acknowledge financial support from the Italian Space Agency, ASI (I/R/037/01), under the project "Cosmologia Osservativa con XMM-Newton''.