Preheating of the early Universe by radiation from high-mass X-ray binaries

TL;DR

This paper estimates how high-mass X-ray binaries in the early Universe could have heated the intergalactic medium through soft X-ray emission, affecting the observability of the 21 cm hydrogen line at high redshifts.

Contribution

It provides a quantitative estimate of the preheating effect of HMXBs on the early Universe's intergalactic medium based on observed luminosity functions.

Findings

X-ray irradiation from HMXBs could heat the IGM by z~10.

Significant heating requires higher X-ray emissivity in early low-metallicity galaxies.

Potential to observe 21 cm emission from z<10 due to this heating.

Abstract

Using a reliably measured intrinsic (i.e. corrected for absorption effects) present-day luminosity function of high-mass X-ray binaries (HMXBs) in the 0.25-2 keV energy band per unit star-formation rate, we estimate the preheating of the early Universe by soft X-rays from such systems. We find that X-ray irradiation, mainly executed by ultraluminous and supersoft ultraluminous X-ray sources with luminosity L> 10^39 erg/s, could significantly heat (T>T_cmb, where T_cmb is the temperature of the cosmic microwave background) the intergalactic medium by z~10 if the specific X-ray emissivity of the young stellar population in the early Universe was an order of magnitude higher than at the present epoch (which is possible due to the low metallicity of the first galaxies) and the soft X-ray emission from HMXBs did not suffer strong absorption within their galaxies. This makes it possible to observe the 21 cm line of neutral hydrogen in emission from redshifts z<10.