XMM-Newton observations of the Lockman Hole IV: spectra of the brightest AGN

This paper presents the results of a detailed X-ray spectral analysis of a sample of 123 X-ray sources detected with XMM-Newton in the Lockman Hole field. This is the deepest observation carried out with XMM-Newton with more that 600 ks of good EPIC-pn data. We have spectra with good signal to noise (>500 source counts) for all objects down to 0.2-12 keV fluxes of ~5 x 10[superscript -15] erg cm[superscript -2] s[superscript -1] (flux limit of ~6 x 10[superscript -16] erg cm[superscript -2] s[superscript -1] in the 0.5-2 and 2-10 keV bands). At the time of the analysis, we had optical spectroscopic identifications for 60% of the sources, 46 being optical type-1 AGN and 28 optical type-2 AGN. Using a single power law model our sources' average spectral slope hardens at faint 0.5-2 keV fluxes but not at faint 2-10 keV fluxes. We have been able to explain this effect in terms of an increase in X-ray absorption at faint fluxes. We did not find in our data any evidence for the existence of a population of faint intrinsically harder sources. The average spectral slope of our sources is ~1.9, with an intrinsic dispersion of ~0.28. We detected X-ray absorption (F-test significance ≥ 95%) in 37% of the sources, ~10% in type-1 AGN (rest-frame N[subscript H] ~ 1.6 x 10[superscript 21] – 1.2 x 10[superscript 22] cm[superscript -2]) and ~ 77% (rest-frame N[subscript H] ~ 1.5 x 10[superscript 21] -4 x 10[superscript 23] cm[superscript-2]) in type-2 AGN. Using X-ray fluxes corrected for absorption, the fraction of absorbed objects and the absorbing column density distribution did not vary with X-ray flux. Our type-1 and type-2 AGN do not appear to have different continuum shapes, but the distribution of intrinsic (rest-frame) absorbing column densities is different among both classes. A significant fraction of our type-2 AGN (5 out of 28) were found to display no substantial absorption (N[subscript H] < 10[superscript 21] cm[superscript -2]). We discuss possible interpretations to this in terms of Compton-thick AGN and intrinsic Broad Line Region properties. An emission line compatible with Fe Kα was detected in 8 sources (1 type-1 AGN, 5 type-2 AGN and 2 unidentified) with rest frame equivalent widths 120-1000 eV. However weak broad components can be easily missed in other sources by the relatively noisy data. The AGN continuum or intrinsic absorption did not depend on X-ray luminosity and/or redshift. Soft excess emission was detected in 18 objects, but only in 9 (including 4 type-1 AGN and 4 type-2 AGN) could we fit this spectral component with a black body model. The measured 0.5-2 keV luminosities of the fitted black body were not significantly different in type-1 and type-2 AGN, although the temperatures of the black body were slightly higher in type-2 AGN (‹kT› = 0.26 ± 0.08) than in type-1 AGN (‹kT› = 0.09 ± 0.01). For 9 sources (including 1 type-1 AGN and 3 type-2 AGN) a scattering model provided a better fit of the soft excess emission. We found that the integrated contribution from our sources to the X-ray background in the 2-7 keV band is softer (Γ = 1.5-1.6) than the background itself, implying that fainter sources need to be more absorbed.