Bosenova collapse of axion cloud around a rotating black hole
  Motivated by possible existence of stringy axions with ultralight mass, we
study the behavior of an axion field around a rapidly rotating black hole (BH)
obeying the sine-Gordon equation by numerical simulations. Due to superradiant
instability, the axion field extracts the rotational energy of the BH and the
nonlinear self-interaction becomes important as the field grows larger. We
present clear numerical evidences that the nonlinear effect leads to a collapse
of the axion cloud and a subsequent explosive phenomena, which is analogous to
the "bosenova" observed in experiments of Bose-Einstein condensate. The
criterion for the onset of the bosenova collapse is given. We also discuss the
reason why the bosenova happens by constructing an effective theory of a
wavepacket model under the nonrelativistic approximation.
