Radiation Backgrounds at Cosmic Dawn: X-Rays from Compact Binaries
  We compute the expected X-ray diffuse background and radiative feedback on
the intergalactic medium (IGM) from X-ray binaries prior and during the epoch
of reionization. The cosmic evolution of compact binaries is followed using a
population synthesis technique that treats separately neutron stars and black
hole binaries in different spectral states and is calibrated to reproduce the
observed X-ray properties of galaxies at z<4. Together with an updated
empirical determination of the cosmic history of star formation, recent
modeling of the stellar mass-metallicity relation, and a scheme for absorption
by the IGM that accounts for the presence of ionized HII bubbles during the
epoch of reionization, our detailed calculations provide refined predictions of
the X-ray volume emissivity and filtered radiation background from "normal"
galaxies at z>6. Radiative transfer effects modulate the background spectrum,
which shows a characteristic peak between 1 and 2 keV. While the filtering of
X-ray radiation through the IGM slightly increases the mean excess energy per
photoionization, it also weakens the radiation intensity below 1 keV, lowering
the mean photoionization and heating rates. Numerical integration of the rate
and energy equations shows that the contribution of X-ray binaries to the
ionization of the bulk IGM is negligible, with the electron fraction never
exceeding 1%. Direct HeI photoionizations are the main source of IGM heating,
and the temperature of the largely neutral medium in between HII cavities
increases above the temperature of the cosmic microwave background (CMB) only
at z<10, when the volume filling factor of HII bubbles is already >0.1.
Therefore, in this scenario, it is only at relatively late epochs that the bulk
of neutral intergalactic hydrogen may be observable in 21-cm emission against
the CMB.
