X-ray Background at High Redshifts from Pop III Remnants: Results from
  Pop III star formation rates in the Renaissance Simulations
  Due to their long mean free paths, X-rays are expected to have many
significant impacts globally on the properties of the intergalactic medium
(IGM) by their heating and ionizing processes on large scales. At high
redshifts, X-rays from Population (Pop) III binaries might have important
effects on cosmic reionization and the Lyman alpha forest. As a continuation of
our previous work on Pop III binary X-rays (Xu et al. 2014), we use the Pop III
distribution and evolution from the Renaissance Simulations, a suite of
self-consistent cosmological radiation hydrodynamics simulations of the
formation of the first galaxies, to calculate the X-ray luminosity density and
background over the redshift range 20 > z > 7.6. As we find that Pop III star
formation continues at a low, nearly constant rate to the end of reionization,
X-rays are being continuously produced at significant rates compared to other
possible X-ray sources, such as AGNs and normal X-ray binaries during the same
period of time. We estimate that Pop III binaries produce approximately 6 eV of
energy in the X-rays per hydrogen atom. We calculate the X-ray background for
different monochromatic photon energies. KeV X-rays redshift and accumulate to
produce a strong X-ray background spectrum extending to roughly 500 eV. The
X-ray background is strong enough to heat the IGM to ~ 1000 K and to ionize a
few percent of the neutral hydrogen. These effects are important for an
understanding of the neutral hydrogen hyperfine transition 21-cm line
signatures, the Ly alpha forest, and optical depth of the CMB to Thomson
scattering.
