2000MNRAS.316..234R

The results from an X-ray spectral analysis of a large sample of quasars, observed with ASCA, are presented. The sample was selected to include all ASCA observations of quasars, with z>0.05 and M_V←23.0, available up to 1998 January. The data reduction leaves 62 quasars, 35 of which are radio-loud and 27 radio-quiet, suitable for spectral analysis. Differences are found between the radio-quiet quasars (RQQs) and the radio-loud quasars (RLQs): the RLQs have flatter X-ray spectra (Γ∼1.6), with little iron line emission or reflection, and are more X-ray luminous than the softer (Γ∼1.9) RQQs, in agreement with previous studies. A correlation between Γ and optical Hβ was also found for the radio-quiet quasars in this sample, whereby the steepest X-ray spectra tend to be found in those objects with small Hβ widths. The correlation is significant at >99 per cent confidence, confirming the well-known trend between Γ and Hβ FWHM in Seyfert 1s, but at higher luminosities.

Other spectral complexities are observed from this sample. A soft X-ray excess, with blackbody temperatures in the range 100-300eV, is seen in many low-z radio-quiet quasars. In most cases the temperatures are probably too hot to originate directly from the disc and could imply that some reprocessing is involved. Iron K-line emission features are also found in the RQQs, but often from partially ionized material. Indeed, in the highest-luminosity RQQs there is neither evidence for iron line emission nor the reflection component expected from disc reflection models. These observations can be explained by an increase in the quasar accretion rate with luminosity, leading to an increase in the ionization state of the surface layers of the disc. The occurrence of ionized or `warm' absorbers is rare in this sample, with only five detections in low-z objects. However, excess neutral X-ray absorption is found towards several of the high-z, predominantly radio-loud, quasars. Although found to increase with quasar redshift, this `intrinsic' absorption may be associated with radio-loud active galactic nuclei.