Extreme mass-ratio inspirals into black holes surrounded by scalar clouds

TL;DR

This paper investigates how scalar clouds around black holes influence extreme mass-ratio inspirals, revealing significant scalar radiation effects and unique signatures like resonances and floating orbits, with implications for gravitational wave detection.

Contribution

It introduces a relativistic framework for analyzing inspirals into black holes with scalar clouds, including new calculations of scalar radiation and its impact on gravitational waves.

Findings

Scalar radiation can dominate over gravitational-wave emission in some regimes.

Resonances and floating orbits are predicted due to scalar cloud interactions.

Corrections to gravitational-wave fluxes can be explained by gravitational redshift effects.

Abstract

We study extreme mass-ratio binary systems in which a stellar mass compact object spirals into a supermassive black hole surrounded by a scalar cloud. Scalar clouds can form through superradiant instabilities of massive scalar fields around spinning black holes and can also serve as a proxy for dark matter halos. Our framework is fully relativistic and assumes that the impact of the cloud on the geometry can be treated perturbatively. As a proof of concept, here we consider a point particle in circular, equatorial motion around a non-spinning black hole surrounded either by a spherically symmetric or a dipolar non-axisymmetric scalar cloud, but the framework can in principle be generalized to generic black hole spins and scalar cloud geometries. We compute the leading-order power lost by the point particle due to scalar radiation and show that, in some regimes, it can dominate over gravitational-wave emission. We confirm the presence of striking signatures due to the presence of a scalar cloud that had been predicted using Newtonian approximations, such as resonances that can give rise to sinking and floating orbits, as well as "sharp features" in the power lost by the particle at given orbital radii. Finally, for a spherically symmetric scalar cloud, we also compute the leading-order corrections to the black-hole geometry and to the gravitational-wave energy flux, focusing only on axial metric perturbations for the latter. We find that, for non-compact clouds, the corrections to the (axial) gravitational-wave fluxes at high frequencies can be understood in terms of a gravitational-redshift effect, in agreement with previous works.