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.
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.
String theory/M-theory generally predicts that axionic fields with a broad mass spectrum extending below 10^{-10}eV are produced after compactification to four dimensions. These axions/fields provoke a rich variety of cosmophysical phenomena on diffe
rent scales depending on their masses and provide us new windows to probe the ultimate theory. In this article, after overviewing this axiverse idea, I take up the black hole instability as the most fascinating one among such axionic phenomena and explain its physical mechanism and astrophysical predictions.
