Growth of accretion driven scalar hair around Kerr black holes

TL;DR

This paper investigates how scalar fields form accretion clouds around Kerr black holes, analyzing their growth, structure, and potential gravitational wave signals in the context of scalar-tensor theories and dark matter models.

Contribution

It provides a detailed study of scalar cloud growth around Kerr black holes with non-trivial boundary conditions, combining non-linear simulations with analytic perturbative analysis.

Findings

Scalar clouds grow from homogeneous initial conditions around Kerr black holes.

The spatial profile of scalar fields depends on black hole spin and scalar field parameters.

Accreted scalar clouds could produce detectable monochromatic gravitational waves.

Abstract

Scalar fields around compact objects are of interest for scalar-tensor theories of gravity and dark matter models consisting of a massive scalar, e.g. axions. We study the behaviour of a scalar field around a Kerr black hole with non trivial asymptotic boundary conditions - both non zero density and non zero angular momentum. Starting from an initial radially homogeneous configuration, a scalar cloud is accreted, which asymptotes to known stationary configurations over time. We study the cloud growth for different parameters including black hole spin, scalar field mass, and the scalar field density and angular momentum far from the black hole. We characterise the transient growth of the mass and angular momentum in the cloud, and the spatial profile of the scalar around the black hole, and relate the results of fully non-linear simulations to an analytic perturbative expansion. We also highlight the potential for these accreted clouds to create monochromatic gravitational wave signals - similar to the signals from superradiant clouds, although significantly weaker in amplitude.