Abstract The cross-correlation between the high-redshift 21 cm background and the Soft X-ray Background (SXB) of the Universe may provide an additional probe of the Epoch of Reionization. Here we use semi-numerical simulations to create 21 cm and soft X-ray intensity maps and construct their cross power spectra. Our results indicate that the cross power spectra are sensitive to the thermal and ionizing states of the intergalactic medium (IGM). The 21 cm background correlates positively to the SXB on large scales during the early stages of the reionization. However as the reionization develops, these two backgrounds turn out to be anti-correlated with each other when more than ~15% of the IGM is ionized in a warm reionization scenario. The anti-correlated power reaches its maximum when the neutral fraction declines to 0.2–0.5. Hence, the trough in the cross power spectrum might be a useful tool for tracing the growth of HII regions during the middle and late stages of the reionization. We estimate the detectability of the cross power spectrum based on the abilities of the Square Kilometre Array and the Wide Field X-ray Telescope (WFXT), and find that to detect the cross power spectrum, the pixel noise of X-ray images has to be at least 4 orders of magnitude lower than that of the WFXT deep survey.

Keywords cosmic background radiation — cosmology: theory — dark ages, reionization, first stars — early Universe — diffuse radiation — X-rays: diffuse background

https://doi.org/10.1088/1674–4527/16/8/132