Dynamical Friction in Gravitational Atoms

TL;DR

This paper investigates how ultralight boson clouds around rotating black holes, called gravitational atoms, influence binary systems' dynamics, revealing effects like increased capture cross sections and orbital circularization, relevant for gravitational wave detection.

Contribution

It provides the first detailed analysis of the dynamical effects of gravitational atoms on binary orbits, including eccentric and inclined cases, enhancing understanding for future gravitational wave searches.

Findings

Cloud presence increases capture cross section by over tenfold.

Ionization causes rapid orbital circularization.

Inclination angle remains largely unaffected by ionization.

Abstract

Due to superradiant instabilities, clouds of ultralight bosons can spontaneously grow around rotating black holes, creating so-called "gravitational atoms". In this work, we study their dynamical effects on binary systems. We first focus on open orbits, showing that the presence of a cloud can increase the cross section for the dynamical capture of a compact object by more than an order of magnitude. We then consider closed orbits and demonstrate that the backreaction of the cloud's ionization on the orbital motion should be identified as dynamical friction. Finally, we study for the first time eccentric and inclined orbits. We find that, while ionization quickly circularizes the binary, it barely affects the inclination angle. These results enable a more realistic description of the dynamics of gravitational atoms in binaries and pave the way for dedicated searches with future gravitational wave detectors.