An X-ray survey of quasars using ASCA

The ASCA X-ray observations of 24 quasars, with redshifts z>0.05, are presented. The greater sensitivity and spectral resolution of ASCA extend the EXOSAT and Ginga surveys to higher redshifts. Significant intrinsic absorption is observed in some of the high-redshift objects, and a strong correlation is seen between absorption and quasar redshift, significant at >99 per cent confidence level. The radio-loud objects in this survey are found, in general, to have flatter X-ray power-law spectra with weak or no iron fluorescence lines. In contrast, the radio-quiet quasars seem to have steeper spectra, with stronger iron lines. The mean photon index of the radio-quiet quasars is found to be 1.93+/-0.06, whereas the radio-loud quasars have a mean index of 1.63+/-0.04. This corresponds to a significant difference in spectral shape between the radio-loud and radio-quiet quasars, at about the 99 per cent significance level, which was not found in the earlier Ginga sample.