Deci-Hz Gravitational Waves from the Self-Interacting Axion Cloud Around a Rotating Stellar Mass Black Hole

Gravitational waves from condensates of ultralight particles, such as axions, around rotating black holes are a promising probe to search for unknown physics. For this purpose, we need to characterize the signal to detect the gravitational waves, which requires tracking the evolution of the condensates, including various effects. The axion self-interaction causes the nonlinear coupling between the superradiant modes, resulting in complicated branching of evolution. Most studies so far have considered evolution under the non- relativistic approximation or the two-mode approximation. In this paper, we numerically investigate the evolution of the axion condensate without these approximations, taking higher multipole modes into account. We also investigate the possible signature in gravitational waves from the condensate. We show that the higher multipole modes are excited, leading to the gravitational wave signal by the transition of the axion between different levels. The most prominent signal of gravitational waves arises from the transition between modes with their angular quantum numbers different by two. The gravitational wave signal is emitted in the deci-Hz band for stellar mass black holes, which might be observable with the proposed gravitational wave detectors.