X-ray spectral properties of active galactic nuclei in the Chandra Deep Field South

We present a detailed X-ray spectral analysis of the sources in the 1Ms catalog of the Chandra Deep Field South ( CDFS) taking advantage of optical spectroscopy and photometric redshifts for 321 extragalactic sources out of the total sample of 347 sources. As a default spectral model, we adopt a power law with slope Gamma with an intrinsic redshifted absorption N-H, a fixed Galactic absorption and an unresolved Fe emission line. For 82 X-ray bright sources, we are able to perform the X-ray spectral analysis leaving both Gamma and N-H free. The weighted mean value for the slope of the power law is <Gamma > similar or equal to 1.75 +/- 0.02, and the distribution of best fit values shows an intrinsic dispersion of sigma(int) similar or equal to 0.30. We do not find hints of a correlation between the spectral index Gamma and the intrinsic absorption column density N-H. We then investigate the absorption distribution for the whole sample, deriving the N-H values in faint sources by fixing Gamma = 1.8. We also allow for the presence of a scattered component at soft energies with the same slope of the main power law, and for a pure reflection spectrum typical of Compton-thick AGN. We detect the presence of a scattered soft component in 8 sources; we also identify 14 sources showing a reflection-dominated spectrum. The latter are referred to as Compton-thick AGN candidates. By correcting for both incompleteness and sampling-volume effects, we recover the intrinsic N-H distribution representative of the whole AGN population, f ( N-H) dN(H), from the observed one. f ( N-H) shows a lognormal shape, peaking around log( N-H) similar or equal to 23.1 and with sigma similar or equal to 1.1. Interestingly, such a distribution shows continuity between the population of Compton-thin and that of Compton-thick AGN. We find that the fraction of absorbed sources ( with N-H > 10(22) cm(-2)) in the sample is constant ( at the level of about 75%) or moderately increasing with redshift. Finally, we compare the optical classification to the X-ray spectral properties, confirming that the correspondence of unabsorbed ( absorbed) X-ray sources to optical type I ( type II) AGN is accurate for at least 80% of the sources with spectral identification ( 1/3 of the total X-ray sample).