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

Abstract

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 ‹αx› and the standard deviation σ for QSOs and NLXGs, assuming the intrinsic distribution to be Gaussian. We find no difference between QSOs and NLXGs: ‹αx› =1.32 with dispersion σ= 0.33 for the QSOs, and ‹αx› = 1.30 with σ = 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 αx with increasing redshift, and for a steepening of αx with increasing (L2500 ÅLx)- For the QSO sample we found no significant correlation. The lack of correlation between αx and z suggests that for the CRSS QSOs the power-law spectrum in the QSO rest-frame extends from the soft ( ∼ 0.1−2.4 keV) into the harder X-ray band ( ∼ 0.3−7.3 keV) with the same slope.