New constraints on the X-ray spectral properties of type 1 active galactic nuclei

We present a detailed characterization of the X-ray spectral properties of 761 type 1 active galactic nuclei (AGN), selected from a cross-correlation of the SDSS DR5 quasar catalogue and the incremental version of the second XMM–Newton serendipitous X-ray source catalogue 2XMMi-DR2. The X-ray spectrum of each source has been fitted with models based on a simple power law to which additional cold absorption and/or soft-excess features have been added if an F-test at 99 per cent significance required them. The distribution of best-fitting photon indices, Γ, has been fitted with a Gaussian with mean 〈Γ〉= 1.99 ± 0.01 and dispersion σ〈Γ〉= 0.30 ± 0.01; however, this does not provide a good representation of the distribution due to sources with very flat or steep Γ values. A highly significant trend for decreasing Γ values with increasing 2–10 keV luminosity, LX, has been seen but only a weak trend with redshift has been found. Intrinsic cold absorption has been detected in ∼4 per cent of the sample and soft-excess emission has been detected in ∼8 per cent. These values are lower limits due to the detectability being limited by the quality of the spectra and we suggest that the intrinsic values may be as high as ∼25 and ∼80 per cent, respectively. The levels of rest-frame absorption are higher than expected for type 1 objects (NH= 1021–1023 cm−2) and the fraction of absorbed sources and the NH values were not seen to vary with LX or z. The average blackbody temperature used to model the soft excesses is 〈kT〉= 0.17 ± 0.09 keV. This temperature has been found to correlate with LX but not the blackbody luminosity or the black hole mass which do correlate with each other. A strong correlation has been found between the luminosities in the blackbody and power-law components, suggesting that a similar fraction is reprocessed from the blackbody to the power-law component for the entire luminosity range of objects. A positive correlation between Γ and the X-ray derived Eddington ratio has been found for the sources whose mass was determined using the Hβ line, but a negative correlation has been found where the C iv line was used. No correlation has been found where the Mg ii line was used. No significant correlations have been found between the blackbody temperature, luminosity or black hole mass with Eddington ratio, despite a link between the power law and blackbody production being indicated. The sample includes 552 confirmed radio-quiet quasars (RQQ) and 75 confirmed radio-loud quasars (RLQ). The RLQ have been found to have higher LX values than their RQQ counterparts, suggesting an additional X-ray component, perhaps related to a jet, is present in these sources. This component may also be the cause of the flatter Γ values seen in RLQ.