Devoured by a Hairy Gargantua: Probing Massive Scalar Charges with Non-minimal Curvature Coupling with Extreme-Mass-Ratio Inspirals

Abstract: Ultralight scalar fields nonminimally coupled to curvature can endow rotating black holes with stationary hair and alter nearby orbits. Using recently constructed hairy black-hole spacetimes, we model the transition to plunge of an extreme-mass-ratio inspiral. For scalar masses $μM=0.01$ and $0.1$ and dimensionless curvature coupling $ζ=10<sup>{-3},10<sup>{-4}$, we find $\mathcal{O}(10)$ rad of gravitational-wave dephasing relative to general relativity over $\mathcal{O}(10<sup>3)$ orbits. A multimode Fisher forecast at $μM=0.2$, where neighbouring hair solutions permit a numerical $μ$-derivative, suggests that $ζ$ and $μ$ could be measured to precisions of $\simeq0.2\%$ and $0.6$--$1.1\%$, respectively, for a source with signal-to-noise ratio of about 80 in LISA.