Dicke's Superradiance in Astrophysics. I -- The 21 cm Line
  We have applied the concept of superradiance introduced by Dicke in 1954 to
astrophysics by extending the corresponding analysis to the magnetic dipole
interaction characterizing the atomic hydrogen 21 cm line. Although it is
unlikely that superradiance could take place in thermally relaxed regions and
that the lack of observational evidence of masers for this transition reduces
the probability of detecting superradiance, in situations where the conditions
necessary for superradiance are met (i.e., close atomic spacing, high velocity
coherence, population inversion, and long dephasing time-scales compared to
those related to coherent behavior), our results suggest that relatively low
levels of population inversion over short astronomical length-scales (e.g., as
compared to those required for maser amplification) can lead to the cooperative
behavior required for superradiance in the ISM. Given the results of our
analysis, we expect the observational properties of 21-cm superradiance to be
characterized by the emission of high intensity, spatially compact, burst-like
features potentially taking place over short periods ranging from minutes to
days.
