  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 inter-galactic 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). In this Letter we use results from detailed, large scale
population synthesis simulations to study the energy feedback of XRBs, from the
first galaxies (z~ 20) until today. We estimate that 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 contrast to its
normalization which evolves by ~4 orders of magnitude, primarily due to the
evolution of the cosmic star-formation rate. However, the metallicity and the
mean stellar age of a given XRB population affect significantly its X-ray
output. Specifically, the X-ray luminosity from high-mass XRBs per unit of
star-formation rate varies an order of magnitude going from solar metallicity
to less than 10% solar, and the X-ray luminosity from low-mass XRBs per unit of
stellar mass peaks at an age of ~300 Myr and then decreases gradually at later
times, showing little variation for mean stellar ages > 3 Gyr. Finally, we
provide analytical and tabulated prescriptions for the energy output of XRBs,
that can be directly incorporated in cosmological simulations.
