We use high resolution hydrodynamical simulations to study the contribution to the X-ray background from high- z energetic sources , such as X-ray binaries , accreting nuclear black holes and shock heated interstellar medium .
Adopting the model discussed in \citet Eide2018 , we find that these X-ray sources during the Epoch of Reionization ( EoR ) contribute less than a few percent of the unresolved X-ray background .
The same sources contribute to less than \sim 2 % of the measured angular power spectrum of the fluctuations of the X-ray background .
The outputs of radiative transfer simulations modeling the EoR are used to evaluate the cross-correlations of X-ray background with the 21 cm signal from neutral hydrogen .
Such correlation could be used to confirm the origin of the 21 cm signal , as well as give information on the properties of the X-ray sources during the EoR .
We find that the correlations are positive during the early stages of reionization when most of the hydrogen is neutral , while they become negative when the intergalactic medium gets highly ionized , with the transition from positive to negative depending on both the X-ray model and the scale under consideration .
With SKA as the reference instrument for the 21 cm experiment , the predicted S/N for such correlations is < 1 if the corresponding X-ray survey is only able to resolve and remove X-ray sources with observed flux > 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } , while the cumulative S/N from l = 1000 to 10 ^ { 4 } at x _ { HI } = 0.5 is \sim 5 if sources with observed flux > 10 ^ { -17 } erg cm ^ { -2 } s ^ { -1 } are detected .