X-ray emission from the first quasars

It is currently unknown whether the universe was reionized by quasars or stars at z ≳ 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 ~10^2 quasars from redshifts z ≳ 5 per its 17' × 17' field of view at the flux threshold of ~2 × 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.