New evidence for Dicke's superradiance in the 6.7 GHz methanol spectral
  line in the interstellar medium
  We present new evidence for superradiance in the methanol 6.7 GHz spectral
line for three different star-forming regions: S255IR-NIRS3, G24.329+0.144, and
Cepheus A. Our analysis shows that some of the intensity flares exhibiting fast
flux rise times and asymmetric light curves reported in these sources can
naturally be explained within the context of superradiance. When a threshold
for the inverted population column density is exceeded in a maser-hosting
region, the radiation mode switches from one regulated by stimulated emission
(maser) to superradiance. Superradiance, as a more efficient energy release
mechanism, manifests itself through strong bursts of radiation emanating from
spatially compact regions. Elevated inverted population densities and the
triggering of superradiance can be due to a change in radiative pumping. Here,
we show that an increase in the pump rate and the inverted population density
of only a factor of a few results in a significant increase in radiation. While
the changes in the pump rate can take place over a few hundred days, the rise
in radiation flux density when superradiance is initiated is drastic and
happens over a much shorter time-scale.
