X-ray Spectra of a large sample of Quasars with ASCA

Abstract

The results from an X-ray spectral analysis of a large sample of quasars, observed with ASCA, are presented. Data reduction leaves 62 quasars, 35 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), in agreement with previous studies. A correlation 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 narrow H-beta widths. The correlation is significant at >99% confidence, confirming the well-known trend between Gamma and H-beta FWHM in Seyfert 1s (Brandt et al. 1997), but at higher luminosities. Other spectral complexities are observed from this sample. A soft X-ray excess, with blackbody 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 directly from the disk. Iron K line emission features are also found in the RQQs; but often from partially ionised material. Indeed in the highest luminosity RQQs there is neither evidence for iron line emission nor the reflection component expected from disk models. These observations can be explained by an increase in the quasar accretion rate with luminosity, leading to an increase in the ionisation state of the surface layers of the disk. The occurrence of ionised or `warm' absorbers is rare in this sample, with only 5 detections. 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 AGN.