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.
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