The XMM-Newton wide-field survey in the COSMOS field. IV. X-ray spectral properties of active galactic nuclei

We present a detailed spectral analysis of pointlike X-ray sources in the XMM-Newton COSMOS field. Our sample of 135 sources only includes those that have more than 100 net counts in the 0.3-10 keV energy band and have been identified through optical spectroscopy. The majority of the sources are well described by a simple power-law model with either no absorption (76%) or a significant intrinsic, absorbing column (20%). The remaining ∼4% of the sources require a more complex modeling by incorporating additional components to the power law. For sources with more than 180 net counts (bright sample), we allowed both the photon spectral index Γ and the equivalent hydrogen column N _H to be free parameters. For fainter sources, we fix Γ to the average value and allow N _H to vary. The mean spectral index of the 82 sources in the bright sample is (Γ) = 2.06 ± 0.08, with an intrinsic dispersion of ∼0.24. Each of these sources has fractional errors on the value of T below 20%. As expected, the distribution of intrinsic absorbing column densities is markedly different between AGNs with or without broad optical emission lines. We find within our sample four type 2 QSO candidates (L _X > 10 ⁴⁴ ergs s ^-1, N _H > 10 ²² cm ^-2), with a spectral energy distribution well reproduced by a composite Seyfert 2 spectrum, that demonstrates the strength of the wide-field COSMOS XMM-Newton survey to detect these rare and underrepresented sources. In addition, we have identified a Compton-thick (N _H > 1.5 × 10 ²⁴ cm ^-2) AGN at z = 0.1248. Its X-ray spectrum is well fitted by a pure reflection model and a significant Fe Kα line at rest-frame energy of 6.4 keV.