X-ray photons, because of their long mean-free paths, can easily escape the galactic environments where they are produced, and interact at long distances with the intergalactic medium, potentially having a significant contribution to the heating and reionization of the early Universe. The two most important sources of X-ray photons in the Universe are active galactic nuclei (AGN) and X-ray binaries (XRBs). The X-ray emission from XRBs dominates over AGN at z ≳ 6 - 8. The shape of the spectral energy distribution of the emission from XRBs shows little change with redshift in constrast to its normalization which evolves by ∽ 4 orders of magnitude, primarly due to the evolution of the cosmic starformation rate.

It is the photons at energy below 2 keV that are most important to early heating and possibly to the reionization of the IGM. What we observe directly for z ≳ 4 galaxies is the rest frame ∽ 3 - 30 keV emission; therefore, the 0.5 - 30 keV spectral energy distribution, measured for local galaxies with Chandra and NuSTAR is a critical input to measuring the X-ray output of high-z galaxies at lower energy.

Recent work, including the deepest Chandra surveies, on the evolution of X-ray emission from galaxies provides strong observational support for the theoretically predicted significant role of XRBs in early heating of the primordial IGM.(Fragos et al. 2013,Basu-Zych et al. 2016, Lehmer et al. 2016)