High-redshift X-ray-selected quasars: CXOCY J125304.0-090737 joins the club

We present a new X-ray-selected high-redshift quasar CXOCY J125304. 0-090737 at z = 4.179, discovered by the Calán-Yale Deep Extragalactic Research (CYDER) Survey. This quasar is the fifth X-ray-selected high-redshift radio-quiet quasar (z > 4) found so far. Here we present its observed properties, which are characterized by its relative optical and X-ray faintness, its X-ray hardness, and its X-ray strength compared with optically selected quasars at high redshift. We also compare the X-ray-selected high-redshift radio-quiet quasars with their optically selected counterparts. We find that the optical to X-ray spectral slope, α_ox is statistically harder (more X-ray luminous) for the X-ray-selected radio-quiet quasars than for the optically selected ones. This result, given the different range of rest frame ultraviolet luminosities studied and the selection of the samples, is consistent with the previously found correlation between X-ray and rest-frame ultraviolet luminosities and would extend that result to a much wider luminosity range at high redshift. Finally, we discuss the prospects of unveiling the quasar luminosity function at high redshifts using X-ray surveys. The discovery of a high-redshift object in the first field of our survey program provides suggestive evidence that X-ray-selected surveys may identify more such objects than would be expected from an extrapolation of the optical luminosity function.