# High Mass X-ray Binaries and the Cosmic 21-cm Signal: Impact of Host Galaxy Absorption

## TL;DR

This study uses high-resolution simulations to assess how host galaxy absorption affects the cosmic 21-cm signal during the Epoch of Heating, revealing that X-ray absorption inside galaxies significantly influences early Universe heating models.

## Contribution

It introduces a detailed calculation of host galaxy X-ray absorption using hydrodynamic simulations, refining predictions of the 21-cm signal during the Epoch of Heating.

## Key findings

- X-ray absorption in host galaxies can be modeled with a metal-free ISM with specific column density.
- The 21-cm power peaks at ~100 mK^2 around redshifts 10-15, depending on star formation history.
- Results support inhomogeneous IGM heating prior to reionization, contrasting with some recent models.

## Abstract

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. 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 / cm^-2] = 21.4 +0.40-0.65. 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 ~ 0.1/Mpc) 21-cm power reaches a peak of ~100 mK^2 at z = 10 - 15, with the redshift depending on the cosmic star formation history. This is in contrast to some recent work, motivated by the much larger X-ray absorption towards local HMXBs inside the Milky Way. Our main results can be reproduced by approximating the X-ray emission from HMXBs with a power-law spectrum with energy index alpha = 1, truncated at energies below 0.5 keV.