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

TL;DR

This study analyzes deep X-ray spectra from the XMM-Newton Lockman Hole survey, revealing insights into AGN absorption properties, spectral characteristics, and the identification of obscured quasars, enhancing understanding of the X-ray universe.

Contribution

It provides the first detailed spectral analysis of a large sample of faint X-ray sources, distinguishing AGN types based on absorption and spectral features.

Findings

No correlation between spectral index and absorption column density.

Mean spectral index ~2 for both Type-1 and Type-2 AGN.

Identification of obscured AGN and estimation of Type-2 QSO density.

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. 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 10^{-15} erg cm^{-2} s^{-1}) of which 61 have redshift identification. We find no correlation between the spectral index Gamma and the intrinsic absorption column density N_H and, for both the Type-1 and Type-2 AGN populations, we obtain <Gamma>~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. About 27% of the subsample with R-K' colour are EROs (R-K>5) and most of these 18 X-ray selected EROs contain anobscured 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.