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

A population of black holes (BHs) at high redshifts (z ≳ 6) that contributes significantly to the ionization of the intergalactic medium (IGM) would be accompanied by the copious production of hard (≳10 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 ≥2 σ level. Distant miniquasars that produce enough X-rays to only partially ionize the IGM to a level of at most xe ~ 50% saturate the unresolved SXB by ≲1 σ. 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.