Constraints on X-ray emissions from the reionization era

Abstract

We examine the constraints on soft X-ray emissions from the reionization era. It has generally been assumed that the Universe was reionized by ultraviolet photons from massive stars. However, it has been argued that X-ray photons associated with the death of these stars would have contributed ~10% to the total ionizations via several channels. The parameter space for a significant component of cosmological reionization to be sourced by X-rays is limited by a few observations. We revisit the unresolved soft X-ray background constraint and show that it significantly limits the contribution to reionization from several potential sources: X-rays from X-ray binaries, from Compton scattering off supernovae-accelerated electrons, and from the annihilation of dark matter particles. We discuss the additional limits on high-redshift X-ray production from (1) z~3 measurements of metal absorption lines, (2) the consensus that helium reionization was ending at z~3, and (3) measurements of the intergalactic medium's thermal history. We show that observations of z~3 metal lines allow little room for extra coeval X-ray emission from nonstandard sources. In addition, we show that the late reionization of helium makes it difficult to also ionize the hydrogen at z>6 with a single source population (such as quasars) and that it likely requires the spectrum of ionizing emissions to soften with increasing redshift. However, it is difficult to constrain an X-ray contribution to reionization from the intergalactic temperature history. We show that the gas would have been heated to a narrower range of temperatures than is typically assumed at reionization, 2-3 x10^4 K.