A Chandra survey of z >= 4.5 quasars

These large scatters can be attributed to the narrow luminosity range at the highest redshift, the large measurement error based on relatively poor X-ray data, and the inclusion of radio-loud quasars in the sample. The L_X_-L_UV_ relationship is significantly sublinear. We do not find a significant redshift evolution of the L_X_-L_UV_ relation, expressed either in the slope of this relation, or the departure of individual AGNs from the best-fitting {alpha}_OX_-L_UV_ relation ({Delta}{alpha}_OX_).

The median value of the X-ray photon index is {Gamma} ~= 1.79, which does not show redshift evolution from z = 0 to z ~ 7. The X-ray and UV properties of the most distant quasars could potentially be used as a standard candle to constrain cosmological models. The large scatter of our sample on the Hubble diagram highlights the importance of future large unbiased deep X-ray and radio surveys in using quasars in cosmological studies.