Mass and spin coevolution of black holes inspiralling through dark matter
Phys. Rev. D 113, 043057 – Published 26 February, 2026
DOI: https://doi.org/10.1103/grjg-5k5w
Abstract
In extreme/intermediate mass-ratio inspirals (E/IMRIs) embedded in dark-matter (DM) spikes, the secondary black hole can accrete collisionless particles from the surrounding halo. We study how the companion’s spin controls this process, and the ensuing backreaction on the magnitude and direction of the companion’s spin vector. We find that higher spin suppresses the mass accretion rate but enhances the accretion-induced torques, driving spin down and secular alignment of the companion’s spin with the orbital plane. Collisionless DM accretion generically imprints a near-universal mass-spin correlation characterized by a spin-evolution parameter , much larger than is the case for typical astrophysical environments, and largely independent of the local DM density and the spike slope. The associated spin down proceeds on astrophysically relevant timescales; thus, observations of rapidly spinning IMRI companions would disfavor the presence of dense DM environments, providing constraints complementary to those arising from dynamical friction.
