A limit from the X-ray background on the contribution of quasars to reionization

A population of black holes (BHs) at high redshifts (zgreater than or similar to6) that contributes significantly to the ionization of the intergalactic medium (IGM) would be accompanied by the copious production of hard (greater than or similar to10 keV) X-ray photons. The resulting hard X-ray background would redshift and be observed as a present-day soft X-ray background (SXB). Under the hypothesis that BHs are the main producers of reionizing photons in the high-redshift universe, we calculate their contribution to the present-day SXB. We find that accreting BHs with a hard spectrum ( be it luminous quasars or their lower mass "miniquasar'' counterparts) could not fully reionize the universe without saturating the unresolved component of the 0.5 - 2 keV SXB at the greater than or equal to2 sigma level. Distant miniquasars that produce enough X-rays to only partially ionize the IGM to a level of at most x(e) similar to 50% saturate the unresolved SXB by less than or similar to1 sigma. Improved determinations of the unresolved component of the SXB can provide a powerful constraint on the contribution of accreting BHs to partial or full reionization.