X-ray spectra of a large sample of quasars with ASCA

The results from an X-ray spectral analysis of a large sample of quasars, o bserved with ASCA, are presented. The sample was selected to include all AS CA 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-l oud and 27 radio-quiet, suitable for spectral analysis. Differences are fou nd between the radio-quiet quasars (RQQs) and the radio-loud quasars (RLQs) : the RLQs have flatter X-ray spectra (Gamma similar to 1.6), with little i ron line emission or reflection, and are more X-ray luminous than the softe r (Gamma similar to 1.9) RQQs, ill agreement with previous studies. A corre lation between Gamma and optical H beta 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 H beta widths. The correlation is significant at >99 per cent confidence, confirming the well-known trend between Gamma a nd H beta FWHM in Seyfert Is, but at higher luminosities. Other spectral co mplexities are observed from this sample. A soft X-ray excess, with blackbo dy temperatures in the range 100-300 eV, is seen in many low-z radio-quiet quasars. In most cases the temperatures are probably too hot to originate d irectly from the disc and could imply that some reprocessing is involved. I ron K-line emission features are also found in the RQQs, but often from par tially ionized material, indeed, in the highest-luminosity RQQs there is ne ither evidence for iron line emission nor the reflection component expected from disc reflection models. These observations can be explained by an inc rease in the quasar accretion rate with luminosity. leading to an increase in the ionization state of the surface layers of the disc. The occurrence o f ionized or 'warm' absorbers is rare in this sample, with only five detect ions in low-z objects. However, excess neutral X-ray absorption is found to wards several of the high-z predominantly radio-loud, quasars. Although fou nd to increase with quasar redshift, this 'intrinsic' absorption may be ass ociated with radio-loud active galactic nuclei.