HERA bound on x-ray luminosity when accounting for population III stars
Hovav Lazare, Debanjan Sarkar, Ely D. Kovetz
Abstract
Recent upper bounds from the Hydrogen Epoch of Reionization Array on the cosmological 21-cm power spectrum at redshifts $z\ensuremath{\approx}8$, 10, have been used to constrain ${L}_{\mathrm{X}<2\text{ }\text{ }\mathrm{keV}}/\mathrm{SFR}$, the soft-band x-ray luminosity measured per unit star formation rate (SFR), strongly disfavoring values lower than $\ensuremath{\approx}{10}^{39.5}\text{ }\text{ }\mathrm{erg}\text{ }{\mathrm{s}}^{\ensuremath{-}1}\text{ }{M}_{\ensuremath{\bigodot}}^{\ensuremath{-}1}\text{ }\text{ }\mathrm{yr}$. 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 ${L}_{\mathrm{X}<2\text{ }\text{ }\mathrm{keV}}/\mathrm{SFR}$ 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 ${L}_{\mathrm{X}<2\text{ }\text{ }\mathrm{keV}}/\mathrm{SFR}$ 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.