1997A&A...327..467L

We have performed optical spectroscopy on 31 quasars and 8 narrow emission line objects identified from X-ray sources in two ROSAT PSPC fields at high galactic latitudes. The broad band and X-ray spectral properties of the quasars have been investigated and are compared with the properties of optically selected quasars observed with ROSAT in one of the two fields. We find that optically luminous objects at high redshifts are relatively underluminous in X-rays and discuss attempts to disentangle the relations of α_oxwith luminosity and redshift. Based on simulations of the quasar samples we show that multivariate regression methods applied to the α_ox(l_opt,z) and α_ox(l_x,z) relations can yield misleading results as α_oxand the luminosities are not statistically independent quantities. For our two samples we find that the α_ox(l,z) relations of the optically and X-ray selected samples are consistent with each other, not confirming previous claims about a difference. In both samples the mean X-ray spectra become flatter with increasing redshift. We conclude that the X-ray spectra of the low redshift objects are dominated by a soft excess component which is shifted out of the PSPC sensitivity window for higher redshifts. From the X-ray spectral and broad band properties in our X-ray selected sample we conclude that intrinsic absorption does not play a role in the quasar sample but in some of the narrow line objects. Our overall results are consistent with the view that both optically and X-ray selected samples are drawn from the same population of objects.