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Models and simulations of the epoch of reionization predict that spectra ofthe 21-cm transition of atomic hydrogen will show a clear fluctuation peak, ata redshift and scale, respectively, that mark the central stage of reionizationand the characteristic size of ionized bubbles. This is based on the assumptionthat the cosmic gas was heated by stellar remnants - particularly X-raybinaries - to temperatures well above the cosmic microwave background at thattime (~ 30 K). Here we show instead that the hard spectra (that is, spectrawith more high-energy photons than low-energy photons) of X-ray binaries makesuch heating ineffective, resulting in a delayed and spatially uniform heatingthat modifies the 21-cm signature of reionization. Rather than looking for asimple rise and fall of the large-scale fluctuations (peaking at severalmillikelvin), we must expect a more complex signal also featuring a distinctminimum (at less than a millikelvin) that marks the rise of the cosmic mean gastemperature above the microwave background. Observing this signal, possiblywith radio telescopes in operation today, will demonstrate the presence of acosmic background of hard X-rays at that early time.

The observable signature of late heating of the Universe during cosmic reionization