Modification of the 21-cm power spectrum by X-rays during the epoch of reionisation

We incorporate a contribution to reionisation from X-rays within analytic and semi-numerical simulations of the 21-cm signal arising from neutral hydrogen during the epoch of reionisation. The relatively long X-ray mean free path (MFP) means that ionisations due to X-rays are not subject to the same density bias as UV ionisations, resulting in a substantive modification of the statistics of the 21-cm signal. We explore the impact that X-ray ionisations have on the power spectrum (PS) of 21-cm fluctuations by varying both the average X-ray MFP and the fractional contribution of X-rays to reionisation. In general, prior to the epoch when the intergalactic medium is dominated by ionised regions (H II regions), X-ray-induced ionisation enhances fluctuations on spatial scales smaller than the X-ray MFP. Conversely, at later times when H II regions dominate, small-scale fluctuations in the 21-cm signal are suppressed by X-ray ionisation. However, our modelling also shows that the modification of the 21-cm signal due to the presence of X-rays is sensitive to the relative scales of the X-ray MFP, and the characteristic size of H II regions. We therefore find that X-rays imprint an epoch and scale dependent signature on the 21-cm PS, whose prominence depends on fractional X-ray contribution. We further show that the presence of X-rays smoothes out the shoulder-like signature of H II regions in the 21-cm PS. For example, a 10% contribution to reionisation from X-rays translates to a 20–30% modulation in the 21-cm PS across the scale of H II regions. We show that the MWA will have sufficient sensitivity to detect this modification of the PS, so long as the X-ray photon MFP falls within the range of scales over which the array is most sensitive (∼ 0.1Mpc). In cases in which this MFP takes a much smaller value, an array with larger collecting area would be required. As a result, an X-ray contribution to reionisation has the potential to substantially complicate analysis of the 21-cm PS. On the other hand, a combination of precision measurements and modelling of the 21-cm PS promises to provide an avenue for investigating the role and contribution of X-rays during reionisation.