Investigating X-ray sources during the epoch of reionization with the 21 cm signal

TL;DR

This study uses radiative transfer simulations to analyze how different energetic sources like X-ray binaries, black holes, and hot interstellar medium affect the 21 cm signal during the epoch of reionization, revealing potential for future observational constraints.

Contribution

It introduces a comparative analysis of various energetic sources' impacts on the 21 cm signal, highlighting differences in heating effects and statistical properties during reionization.

Findings

ISM heating increases the global 21 cm signal and reduces fluctuations.

XRBs have a similar but delayed and weaker impact compared to ISM.

Differences between source scenarios exceed SKA1-low noise at high redshifts.

Abstract

Heating of neutral gas by energetic sources is crucial for the prediction of the 21 cm signal during the epoch of reionization (EoR). To investigate differences induced on statistics of the 21 cm signal by various source types, we use five radiative transfer simulations which have the same stellar UV emission model and varying combinations of more energetic sources, such as X-ray binaries (XRBs), accreting nuclear black holes (BHs) and hot interstellar medium emission (ISM). We find that the efficient heating from the ISM increases the average global 21~cm signal, while reducing its fluctuations and thus power spectrum. A clear impact is also observed in the bispectrum in terms of scale and timing of the transition between a positive and a negative value. The impact of XRBs is similar to that of the ISM, although it is delayed in time and reduced in intensity because of the less efficient heating. Due to the paucity of nuclear BHs, the behaviour of the 21~cm statistics in their presence is very similar to that of a case when only stars are considered, with the exception of the latest stages of reionization, when the effect of BHs is clearly visible. We find that differences between the source scenarios investigated here are larger than the instrumental noise of SKA1-low at $z \gtrsim 7-8$, suggesting that in the future it might be possible to constrain the spectral energy distribution of the sources contributing to the reionization process.