The Chandra-COSMOS survey - IV. X-ray spectra of the bright sample

We present the X-ray spectral analysis of the 390 brightest extragalactic sources in the Chandra-Cosmic Evolution Survey catalogue, showing at least 70 net counts in the 0.5-7 keV band. This sample has a 100 per cent completeness in optical-infrared identification, with ∼75 per cent of the sample having a spectroscopic redshift and ∼25 per cent a photometric redshift. Our analysis allows us to accurately determine the intrinsic absorption, the broad-band continuum shape (Γ) and intrinsic L_2-10 distributions, with an accuracy better than 30 per cent on the spectral parameters for 95 per cent of the sample. The sample is equally divided in type 1 (49.7 per cent) and type 2 active galactic nuclei (48.7 per cent) plus few passive galaxies at low z. We found a significant difference in the distribution of Γ of type 1 and type 2, with small intrinsic dispersion, a weak correlation of Γ with L_2-10 and a large population (15 per cent of the sample) of high luminosity, highly obscured (QSO2) sources. The distribution of the X-ray/Optical flux ratio (Log(F_X/F_i)) for type 1 is narrow (0 < X/O < 1), while type 2 are spread up to X/O = 2. The X/O correlates well with the amount of X-ray obscuration. Finally, a small sample of Compton-thick candidates and peculiar sources is presented. In the appendix, we discuss the comparison between Chandra and XMM-Newton spectra for 280 sources in common. We found a small systematic difference, with XMM-Newton spectra that tend to have softer power laws and lower obscuration.