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 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 {\tt 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 $>10^{-15}\mathrm{erg}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$, while the cumulative S/N from $l=1000$ to $10^4$ at $x_{\mathrm{HI}}=0.5$ is $\sim 5$ if sources with observed flux $>10^{-17}\mathrm{erg}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$ are detected.

arXiv comment9 pages, 8 figures

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• Investigating X-ray sources during the epoch of reionization with the 21 cm signal
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• Exploring the cosmic dawn and epoch of reionization with 21cm line
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• Predictions for the 21cm-galaxy cross-power spectrum observable with SKA and future galaxy surveys
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• A CRASH simulation of the contribution of binary stars to the epoch of reionization
• Observing the redshifted 21 cm signal around a bright QSO at $z\sim 10$
• Estimation of the detected background by the future gamma-ray transient mission CAMELOT
• Mocking Faint Black Holes during Reionization
• Probing the Inflaton Potential with SKA
• Nearest Neighbour-Based Statistics for 21cm-Galaxy Cross-Correlations in the Epoch of Reionization