Gravitational Waves from Superradiant Cloud Level Transition

Abstract: 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 can be detected with future gravitational wave detectors such as DECIGO and BBO, providing a potential way of searching for ultralight boson fields. 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 LISA-like detectors.