The Cambridge-Cambridge ROSAT serendipity survey .4. The X-ray properties

We present a detailed X-ray spectral analysis in the 0.1-2.4 keV ROSAT band of a complete sample of X-ray-selected AGN using the 80 AGNs in the Cambridge-Cambridge ROSAT Serendipity Survey (68 QSOs and 12. narrow-emission-line galaxies, NLXGs), We also make a comparison between the X-ray spectral properties of QSOs and NLXGs, For 36 objects we have enough net counts to allow an X-ray spectral fit, while for the other sources we characterize the spectrum using the hardness-ratio technique. A maximum-likelihood analysis is used to find the mean power-law energy spectral index [alpha(x)] and the standard deviation sigma for QSOs and NLXGs, assuming the intrinsic distribution to be Gaussian. We find no difference between QSOs and NLXGs: [alpha(x)] = 1.32 with dispersion sigma = 0.33 for the QSOs, and [alpha(x)] = 1.30 with sigma = 0.49 for the NLXGs. A single power law with a Galactic absorbing column density yields a good representation of the X-ray spectra for the majority of the sources, Only three objects show a significant deviation from this model, There is evidence in the NLXG sample for a flattening of the spectral slope alpha(x) with increasing redshift, and for a steepening of alpha(x) with increasing (L(2500 Angstrom)/L(x)). For the QSO sample we found no significant correlation. The lack of correlation between alpha(x) and z suggests that for the CRSS QSOs the power-law spectrum in the QSO rest-frame extends from the soft (similar to 0.1-2.4 keV) into the harder X-ray band (similar to 0.3-7.3 keV) with the same slope.