HERA bound on x-ray luminosity when accounting for population III stars

TL;DR

This paper revises constraints on the soft X-ray luminosity per star formation rate during cosmic dawn by including Population III stars, showing lower luminosities are not strongly disfavored when accounting for these early stars.

Contribution

It introduces a method incorporating Population III stars into 21-cm signal models, updating previous bounds on X-ray luminosity from HERA data.

Findings

Lower X-ray luminosities are compatible with HERA data when PopIII stars are included.

Including PopIII stars relaxes previous constraints on early X-ray sources.

The revised bounds align high-redshift X-ray sources with observed low-redshift X-ray binaries.

Abstract

Recent upper bounds from the Hydrogen Epoch of Reionization Array (HERA) on the cosmological 21-cm power spectrum at redshifts $z \approx 8, 10$, have been used to constrain $L_{\rm X<2 \, keV}/{\rm SFR}$, the soft-band X-ray luminosity measured per unit star formation rate (SFR), strongly disfavoring values lower than $\approx 10^{39.5} \, {\rm erg} \;{\rm s}^{-1} \;{\rm M}_{\odot}^{-1} \;{\rm yr}$. This conclusion is derived from semi-numerical models of the 21-cm signal, specifically focusing on contributions from atomic cooling galaxies that host PopII stars. In this work, we first reproduce the bounds on $L_{\rm X<2 \, keV}/{\rm SFR}$ and other parameters using a pipeline that combines machine learning emulators for the power spectra and the intergalactic medium characteristics, together with a standard Markov chain Monte Carlo parameter fit. We then use this approach when including molecular cooling galaxies that host PopIII stars in the cosmic dawn 21-cm signal, and show that lower values of $L_{\rm X<2 \, keV}/{\rm SFR}$ are hence no longer strongly disfavored. The revised HERA bound does not require high-redshift X-ray sources to be significantly more luminous than high-mass X-ray binaries observed at low redshift.