X-ray emission from the first quasars

It is currently unknown whether the universe was reionized by quasars or stars at z greater than or similar to 5. We point out that quasars can be best distinguished from stellar systems by their X-ray emission. Based on a simple hierarchical CDM model, we predict the number counts and X-ray fluxes of quasars at high redshifts. The model is consistent with available data on the luminosity function of high-redshift quasars in the optical and soft X-ray bands. The cumulative contribution of faint, undetected quasars in our model is consistent with the unresolved fraction of the X-ray background. We find that the Chandra X-ray Observatory might detect similar to 10(2) quasars from redshifts z greater than or similar to 5 per its 17' x 17' field of view at the flux threshold of similar to 2 x 10(-16) ergs s(-1) cm(-2). The redshifts of these faint point sources could be identified by follow-up infrared observations from the ground or with the Next Generation Space Telescope.