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

Abstract

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 × 10 −15 erg cm −2 s −1 (flux limit of ∼6 × 10 −16 erg cm −2 s −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 H ∼ 1.6 × 10 21 −1.2 × 10 22 cm −2 ) and ∼77% (rest-frame N H ∼ 1.5 × 10 21 −4 × 10 23 cm −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 H < 10 21 cm −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.

The XMM-NewtonWide Angle Survey (XWAS): the X-ray spectrum of type-1 AGN

“…The co-addition of spectra in such a way blurs narrow features (such as iron line emission), however this is not a problem for our analysis as we are interested only in measuring broad band spectral features. The co-added spectra were obtained following the recipe in Appendix A in Mateos et al (2005b). At high energies the S/N of the spectra decreases substantially and the uncertainties associated with background subtraction become much larger.…”

“…Mainieri et al 2007;Tozzi et al 2006;Mateos et al 2005b). This hard X-ray spectral component is believed to originate in hot plasma surrounding the accretion disk which Compton up-scatters UV-soft X-ray thermal emission from the disk into the hard X-ray band (Haardt & Maraschi 1991).…”

“…Spectral analyses have shown that the optical type vs. X-ray properties for most AGN generally agree with the predictions from AGN unification models. However, in recent years, the number of cases where the predictions from AGN unification models do not match the observations has increased substantially, as significant absorption has been detected in ∼10% type-1 AGN (Cappi et al 2006;Mateos et al 2005b;Akylas et al 2004) while type-2 AGN have been detected without any signatures of X-ray absorption (Panessa et al 2009;Bianchi et al 2008;Corral et al 2005;Panessa & Bassani 2002).…”

“…Various studies of the X-ray photon index have produced different results. Some authors claim an anti-correlation between the spectral photon index Γ and the X-ray luminosity (Young et al 2009;Saez et al 2008;Page et al 2005) while other studies suggest the opposite correlation (Dai et al 2004) or no correlation at all (Winter et al 2009;Mateos et al 2005b;George et al 2000;Reeves & Turner 2000). Studies of the X-ray spectra of AGN have generally shown no clear dependence of the AGN X-ray properties on the redshift (Mateos et al 2005a), although some exceptions have been found (Kelly et al 2007).…”

Statistical evaluation of the flux cross-calibration of the XMM-Newton EPIC cameras

“…Indeed even the distribution of broad band continuum shapes is known to have an intrinsic dispersion ΔΓ ∼ 0.2−0.3 (see e.g. Mateos et al 2005). Furthermore, at soft X-ray energies and bright fluxes the contribution from non-AGN populations to the X-ray sky, mainly stars and clusters of galaxies with thermal spectra, is not negligible (Mateos et al 2008).…”