The X-ray properties of high-redshift quasi-stellar objects

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Abstract

Sensitive X-ray information is presented for about 90 previously uncataloged high-redshift, optically selected QSOs. Two high-redshift subsets of this larger data base are emphasized: (1) a 'complete' sample of 37 objects with z = 1.8 - 3.0, and B of 19.5 or less and (2) a sample of 78 QSOs that span a narrow range in redshift and optical luminosity. From the complete sample, it is inferred that such QSOs contribute less than 3 percent to the diffuse X-ray background (XRB), and that the contribution of all QSOs with B of 19.5 or less to the XRB is 21 + or - 8 percent. For the narrow sample, it is found that the mean X-ray to optical luminosity ratio is characterized by alpha(eff)ox = 1.50 + or - 0.03. A variety of new, model-independent, empirical evidence is presented that the primary dependence of alpha(ox) is on optical luminosity rather than redshift. It is concluded that QSOs can supply as much as 70 percent of the XRB, with the typical contributor a moderate redshift QSO with B of roughly 19-21.

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