High-mass X-ray binaries and the cosmic 21-cm signal: impact of host galaxy absorption

By heating the intergalactic medium (IGM) before reionization, X-rays are expected to play a prominent role in the early Universe. The cosmic 21-cm signal from this epoch of heating (EoH) could serve as a clean probe of high-energy processes inside the first galaxies. Here, we improve on prior estimates of this signal by using high-resolution hydrodynamic simulations to calculate the X-ray absorption due to the interstellar medium (ISM) of the host galaxy, typically residing in haloes with mass 10(7.5-8.5) M-circle dot at z similar to 8-15. X-rays absorbed inside the host galaxy are unable to escape into the IGM and contribute to the EoH. We find that the X-ray opacity through these galaxies can be approximated by a metal-free ISM with a typical column density of log[N-Hi/cm(-2)]=21.4(-0.65)(+0.40) . We compute the resulting 21-cm signal by combining these ISM opacities with public spectra of high-mass X-ray binaries (thought to be important X-ray sources in the early Universe). Our results support standard scenarios in which the X-ray heating of the IGM is inhomogeneous, and occurs before the bulk of reionization. The large-scale (k similar to 0.1 Mpc(-1)) 21-cm power reaches a peak of approximate to 100 mK(2) at z similar to 10-15, with the redshift depending on the cosmic star formation history. Our main results can be reproduced by approximating the X-ray emission from high-mass X-ray binaries by a power law with energy index alpha approximate to 1, truncated at energies below 0.5 keV.