Abstract
X-ray binaries (XRBs) are thought to regulate cosmic thermal and ionisation
histories during the Epoch of Reionisation and Cosmic Dawn ($z\sim 5-30$).
Theoretical predictions of the X-ray emission from XRBs are important for
modeling such early cosmic evolution. Nevertheless, the contribution from
Be-XRBs, powered by accretion of compact objects from decretion disks around
rapidly rotating O/B stars, has not been investigated systematically. Be-XRBs
are the largest class of high-mass XRBs (HMXBs) identified in local
observations and are expected to play even more important roles in metal-poor
environments at high redshifts. In light of this, we build a physically
motivated model for Be-XRBs based on recent hydrodynamic simulations and
observations of decretion disks. Our model is able to reproduce the observed
population of Be-XRBs in the Small Magellanic Cloud with appropriate initial
conditions and binary stellar evolution parameters. We derive the X-ray output
from Be-XRBs as a function of metallicity in the (absolute) metallicity range
$Z\in [10^{-4},0.03]$. We find that Be-XRBs can contribute a significant
fraction ($\sim 60\%$) of the total X-ray budget from HMXBs observed in nearby
galaxies for $Z\sim 0.0003-0.02$. A similar fraction of observed ultra-luminous
($\gtrsim 10^{39}\ \rm erg\ s^{-1}$) X-ray sources can also be explained by
Be-XRBs. Moreover, the predicted metallicty dependence in our fiducial model is
consistent with observations, showing a factor of $\sim 8$ increase in X-ray
luminosity per unit star formation rate from $Z=0.02$ to $Z=0.0003$.