  Gravitational waves (GWs) are an exciting new probe of physics beyond the
standard models of gravity and particle physics. One interesting possibility is
provided by the so-called "gravitational atom," wherein a superradiant
instability spontaneously forms a cloud of ultralight bosons around a rotating
black hole. The presence of these boson clouds affects the dynamics of black
hole binary inspirals and their associated GW signals. In this Letter, we show
that the binary companion can induce transitions between bound and unbound
states of the cloud, effectively "ionizing" it, analogous to the photoelectric
effect in atomic physics. The orbital energy lost in this process can overwhelm
the losses due to GW emission, so that ionization drives the inspiral rather
than merely perturbing it. We show that the ionization power contains sharp
features that lead to distinctive "kinks" in the evolution of the emitted GW
frequency. These discontinuities are a unique signature of the boson cloud and
observing them would not only constitute a detection of the ultralight boson
itself, but also provide direct information about its mass and the state of the
cloud.
