Gravitational waves from superradiant cloud level transition

Ultralight boson clouds can form around black holes in binaries through superradiance, and undergo resonant level transitions at certain orbit frequencies. In this work, we investigate the gravitational waves emitted by the clouds during resonant level transitions, and forecast their detectability with future gravitational wave observations. We find that, for scalar fields of mass around $10^{-12}$ eV, clouds in stellar mass black hole binaries can radiate gravitational waves around $0.1$ Hz during hyperfine level transition, that could be detected with future gravitational wave detectors such as Big Bang Observer(BBO), but at a very low event rate. We also consider the clouds in intermediate mass black hole binaries, which can emit milli-Hz gravitational waves during hyperfine level transition. The resulting gravitational waves, however, can be hardly detected with Laser Interferometer Space Antenna(LISA)-like detectors.