Ionization of Gravitational Atoms

TL;DR

This paper explores how ultralight boson clouds around rotating black holes interact with binary companions, revealing ionization and accretion effects that influence gravitational wave signals and can help detect gravitational atoms.

Contribution

It introduces the concept of ionization of gravitational atoms, analyzing how bound-to-unbound transitions and accretion affect binary dynamics and gravitational wave signatures.

Findings

Ionization can dominate over gravitational wave losses in certain conditions.

Impending accretion onto black hole companions significantly alters binary evolution.

Characteristic features in gravitational waves can reveal the energy spectrum of the cloud.

Abstract

Superradiant instabilities may create clouds of ultralight bosons around rotating black holes, forming so-called "gravitational atoms." It was recently shown that the presence of a binary companion can induce resonant transitions between bound states of these clouds, whose backreaction on the binary's orbit leads to characteristic signatures in the emitted gravitational waves. In this work, we show that the interaction with the companion can also trigger transitions from bound to unbound states of the cloud -- a process that we refer to as "ionization" in analogy with the photoelectric effect in atomic physics. The orbital energy lost in the process overwhelms the losses due to gravitational wave emission and contains sharp features carrying information about the energy spectrum of the cloud. Moreover, we also show that if the companion is a black hole, then the part of the cloud impinging on the event horizon will be absorbed. This "accretion" leads to a significant increase of the companion's mass, which alters the dynamical evolution and ensuing waveform of the binary. We argue that a combined treatment of resonances, ionization, and accretion is crucial to discover and characterize gravitational atoms with upcoming gravitational wave detectors.