Gravitational atoms in black hole binaries

Superradiant instabilities may create clouds of ultralight bosons around rotating black holes, forming so-called "gravitational atoms". The presence of a binary companion can induce transitions between bound states of the cloud ("resonances"), as well as from bound to unbound states ("ionization"). These processes backreact on the binary dynamics and leave characteristic imprints on the emitted GWs, carrying direct information about the mass of the boson and the state of the cloud. Some of the resonances might however destroy the cloud before the binary enters the frequency band of (future) gravitational wave detectors — by doing so, they leave a mark on the binary's eccentricity and inclination, which can be found with a statistical analysis of a population of binary black holes.