Recognizing the First Radiation Sources Through Their 21-cm Signature
  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, Pop 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. Further, 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. 
Leonid Chuzhoy, Marcelo A. Alvarez and Paul R. Shapiro