Floating and sinking: the imprint of massive scalars around rotating black holes

TL;DR

It is shown that matter can hover into "floating orbits" for which the net gravitational energy loss at infinity is entirely provided by the black hole's rotational energy, and these effects could be a smoking gun of deviations from general relativity.

Abstract

We study the coupling of massive scalar fields to matter in orbit around rotating black holes. It is generally expected that orbiting bodies will lose energy in gravitational waves, slowly inspiralling into the black hole. Instead, we show that the coupling of the field to matter leads to a surprising effect: because of superradiance, matter can hover into "floating orbits" for which the net gravitational energy loss at infinity is entirely provided by the black hole's rotational energy. Orbiting bodies remain floating until they extract sufficient angular momentum from the black hole, or until perturbations or nonlinear effects disrupt the orbit. For slowly rotating and nonrotating black holes floating orbits are unlikely to exist, but resonances at orbital frequencies corresponding to quasibound states of the scalar field can speed up the inspiral, so that the orbiting body "sinks". These effects could be a smoking gun of deviations from general relativity.

Figures (2)

• Figure 1: Pictorial description of floating orbits. An orbiting body excites superradiant scalar modes close to the BH horizon. Since the scalar field is massive, the flux at infinity consists solely of gravitational radiation.
• Figure 2: Dominant fluxes of scalar and gravitational energy ($l=m=1$ and $l=m=2$, respectively) for $\mu_s M=10^{-2}$, $\alpha=10^{-2}$ and $a=0.99M$. The inset is a zoom around resonance.