The Cambridge-Cambridge ROSAT Serendipity Survey - IV. The X-ray properties

We present a detailed X-ray spectral analysis in the 0.1-2.4keV 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_2500A/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 (~0.1-2.4keV) into the harder X-ray band (~0.3-7.3keV) with the same slope.