AN X-RAY SURVEY OF QUASARS USING ASCA

The ASCA X-ray observations of 24 quasars, with redshifts z > 0.05, ar e presented. The greater sensitivity and spectral resolution of ASCA e xtend the EXOSAT and Ginga surveys to higher redshifts. Significant in trinsic absorption is observed in some of the high-redshift objects, a nd a strong correlation is seen between absorption and quasar redshift , significant at > 99 per cent confidence level. The radio-loud object s in this survey are found, in general, to have flatter X-ray power-la w spectra with weak or no iron fluorescence lines. In contrast, the ra dio-quiet quasars seem to have steeper spectra, with stronger iron lin es, 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 sha pe between the radio-loud and radio-quiet quasars, at about the 99 per cent significance level, which was not found in the earlier Ginga sam ple.