Pre-Proof Draft: NOT Final High Redshift X-Ray Selected Quasars: CXOCY J125304.0-090737 joins the club

We present a new X-ray selected high redshift quasar CXOCY J125304.0090737 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 and also the second faintest. Here, we present its observed properties which are characterized by its relative faintness, hardness and X-ray strength compared to optically selected quasars at high redshift. We also compare the X-ray selected high redshift radio quiet quasars to their optically selected counterparts. We find that the X-ray to optical spectral slope αox is statistically harder (more X-ray luminous) for the X-ray selected radio quiet quasars than for the optically selected ones and similar to the values at lower redshift. This argues against evolution in the αox value, but is consistent with an αox steepening with luminosity. It may also be due to inherent selection biases. 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. Astronomy Department, Yale University, P.O. Box 208101, New Haven, CT 06520 Departamento de Astronomı́a, Universidad de Chile, Casilla 36-D, Santiago, Chile Andes Prize Fellow Current address: Institut d’Estudis Espacials de Catalunya/CSIC, Gran Capità 2-4, 08034 Barcelona, Spain SISSA, via Beirut 2-4, 34014 Trieste, Italy Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824 Department of Astronomy, University of Florida, P.O. Box 112055, Gainesville, FL 32611