New constraints on the X-ray spectral properties of type 1 AGN
  We present a detailed characterization of the X-ray spectral properties of 761 type 1 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 is fit with models based on a simple power law to which additional cold absorption and/or soft excess features are added if an F-test at 99% significance requires them. The distribution of best-fitting photon indices, Gamma, is fit with a Gaussian with mean <gamma> = 1.99 +/- 0.01 and dispersion sigma = 0.30 +/- 0.01, however this does not provide a good representation of the distribution due to sources with very flat or steep gamma values. A highly significant trend for decreasing gamma values with increasing 2-10 keV luminosity, L_X, is seen but only a weak trend with redshift is found. Intrinsic cold absorption is detected in ~4% of the sample and soft excess emission is detected in ~8%. These values are lower limits due to the detectability being limited by the quality of the spectra and we suggest the intrinsic values may be as high as ~25% and ~80% respectively. The levels of rest-frame absorption are higher than expected for type 1 objects (N_H = 10^21-10^23 cm^-2) and the fraction of absorbed sources and the N_H values are not seen to vary with L_X or z. The average blackbody temperature used to model the soft excesses is <kT> = 0.17 +/- 0.09 keV. This temperature is found to correlate with L_X but not the blackbody luminosity or the black hole mass which do correlate with each other. A strong correlation is found between the luminosities in the blackbody and power law components suggesting that a similar fraction is re-processed from the blackbody to the power law component for the entire luminosity range of objects. . . [Abridged] 
A.E. Scott, G.C. Stewart, S. Mateos, D.M. Alexander, S. Hutton and   M.J. Ward
astro-ph.HE astro-ph.CO