  We report some new interesting features of the dynamics of a string axion
field (i.e., a (pseudo-)scalar field with tiny mass with sine-Gordon-type
self-interaction) around a rotating black hole in three respects. First, we
revisit the calculation of the growth rate of superradiant instability, and
show that in some cases, overtone modes have larger growth rates than the
fundamental mode with the same angular quantum numbers when the black hole is
rapidly rotating. Next, we study the dynamical evolution of the scalar field
caused by the nonlinear self-interaction, taking attention to the dependence of
the dynamical phenomena on the axion mass and the modes. The cases in which two
superradiantly unstable modes are excited simultaneously are also studied.
Finally, we report on our preliminary simulations for gravitational wave
emission from the dynamical axion cloud in the Schwarzschild background
approximation. Our result suggests that fairly strong gravitational wave burst
is emitted during the bosenova, which could be detected by the ground-based
detectors if it happens in Our Galaxy or nearby galaxies.
