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

TL;DR

This paper investigates how X-ray ionizations influence the 21-cm power spectrum during reionization, revealing scale-dependent signatures that depend on X-ray properties and the epoch, with implications for detection by radio arrays.

Contribution

It introduces a model incorporating X-ray effects into 21-cm signal simulations, analyzing their impact on the power spectrum across different scales and epochs.

Findings

X-ray ionization enhances small-scale fluctuations before HII regions dominate.

X-ray ionization suppresses small-scale fluctuations when H2 regions are prevalent.

MWA can detect X-ray induced modifications if the X-ray MFP is within certain scales.

Abstract

We incorporate a contribution to reionization from X-rays within analytic and semi-numerical simulations of the 21-cm signal arising from neutral hydrogen during the epoch of reionization. We explore the impact that X-ray ionizations 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 reionization. In general, prior to the epoch when the intergalactic medium is dominated by ionized regions (H {\sevensize II} regions), X-ray-induced ionization enhances fluctuations on spatial scales smaller than the X-ray MFP, provided that X-ray heating does not strongly supress galaxy formation. Conversely, at later times when \H2 regions dominate, small-scale fluctuations in the 21-cm signal are suppressed by X-ray ionization. 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 \H2 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. The degree of X-ray heating of the IGM also determines the extent to which these features can be discerned. 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 ($\sim0.1$ Mpc$^{-1}$). In cases in which this MFP takes a much smaller value, an array with larger collecting area would be required.