The authors study a sample of 178 optically selected quasars and Seyfert galaxies, 77 of which have measured X-ray fluxes while the remainder have upper limits. The sample includes 23 new high-redshift (z > 1), "grism-selected" quasars from two fields. The authors observed each field with the IPC on the Einstein Observatory and detected X-rays from nine of the quasars. The X-ray luminosities of these objects lie in the range (1 - 9)×10⁴⁵ergs s⁻¹. These results show that "typical" radio-quiet quasars are indeed powerful X-ray sources. The authors investigate the distribution function describing the dependence of the X-ray-to-optical luminosity ratio on redshift and optical luminosity with both nonparametric and parametric methods. The analysis confirms the conclusion that the X-ray-to-optical luminosity ratio decreases with increasing optical luminosity. The dependence of the ratio on look-back time is smaller and may be nonexistent. It is also shown that the properties of a complete sample of X-ray selected quasars are not commensurate with the present results for optically selected quasars.
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Kriss et al. (1985) studied this question.