Haiman, ZoltanLoeb, Abraham2019-09-241999Haiman, Zoltan, and Abraham Loeb. 1999. “X-Ray Emission from the First Quasars.” The Astrophysical Journal 521 (1): L9–12. https://doi.org/10.1086/312177.0004-637X1538-4357http://nrs.harvard.edu/urn-3:HUL.InstRepos:41393256It 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.en-USJournal Article2019-09-2410.1086/312177