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

Publisher's version (útgefin grein)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 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.The authors are grateful to James Aird, Andrea Merloni, Arne Rau, and Mara Salvato for inputs on planned X-ray surveys, Ruben Salvaterra for useful comments, and an anonimous referee for his or her comments. The tools for bibliographic research are offered by the NASA Astrophysics Data Systems and by the JSTOR archive. QM is supported by the National Basic Research Program (“973” Program) of China (grant No. 2014CB845800), the National Natural Science Foundation of China (grant Nos 11673068 and 11725314), the Youth Innovation Promotion Association (2011231), the Key Research Program of Frontier Sciences (QYZDB-SSW-SYS005), the Strategic Priority Research Program ‘Multiwaveband gravitational wave Universe’ (grant No. XDB23000000) of the Chinese Academy of Sciences. MBE thanks the Astronomy and Astrophysics department at UCSC for their kind hospitality and is a fellow of the U.S.-Norway Fulbright Foundation. KH acknowledges support from the Icelandic Research Fund, grant number 173728-051.Peer Reviewe