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

Recent upper bounds from the Hydrogen Epoch of Reionization Array on the cosmological 21-cm power spectrum at redshifts z≈8, 10, have been used to constrain LX<2 keV/SFR, the soft-band x-ray luminosity measured per unit star formation rate (SFR), strongly disfavoring values lower than ≈1039.5 erg s-1 M⊙-1 yr. This conclusion is derived from seminumerical models of the 21-cm signal, specifically focusing on contributions from atomic cooling galaxies that host population II stars. In this work, we first reproduce the bounds on LX<2 keV/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 population III stars in the cosmic dawn 21-cm signal, and show that lower values of LX<2 keV/SFR are hence no longer strongly disfavored. The revised Hydrogen Epoch of Reionization Array bound does not require high-redshift x-ray sources to be significantly more luminous than high-mass x-ray binaries observed at low redshift.