At the beginning of the reionization epoch, radiation sources produce fluctuations in the redshifted 21 cm background. We show that different types of sources (such as miniquasars, Population Ⅱ and Ⅲ stars, supernovae, etc.) produce distinct signatures in the 21 cm signal radial profiles and statistical fluctuations, through which they can be identified. Furthermore, we show that the 21 cm signal from X-ray-emitting sources is much(easier to observe than was expected, due to a previously neglected pumping mechanism.
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