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