X-ray background and its correlation with the 21 cm signal

Abstract

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 Eide et al. (2018), we find that these X-ray sources during the Epoch of Reionization (EoR) contribute less than a few per cent of the unresolved X-ray background. The same sources contribute to less than ∼2 per cent 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 >10−15ergcm−2s−1⁠, while the cumulative S/N from l = 1000 to 104 at xHI=0.5 is ∼5 if sources with observed flux >10−17ergcm−2s−1 are detected.