Tidal heating and torquing of a Kerr black hole to next-to-leading order in the tidal coupling

TL;DR

This paper analytically calculates the effects of external gravitational fields on a Kerr black hole, including tidal heating and torquing, to third order in a slow-motion, small-hole approximation, applicable to black hole binaries.

Contribution

It provides the first third-order analytical expressions for tidal interactions of Kerr black holes in the slow-motion regime, extending previous second-order results.

Findings

Derived generic formulas for mass and angular momentum fluxes.

Applied formulas to specific black hole scenarios.

Quantified tidal heating and torquing effects.

Abstract

We calculate the linear vacuum perturbations of a Kerr black hole surrounded by a slowly-varying external spacetime to third order in the ratio of the black-hole mass to the radius of curvature of the external spacetime. This expansion applies to two relevant physical scenarios: (i) a small Kerr black hole immersed in the gravitational field of a much larger external black hole, and (ii) a Kerr black hole moving slowly around another external black hole of comparable mass. This small-hole/slow-motion approximation allows us to parametrize the perturbation through slowly-varying, time-dependent electric and magnetic tidal tensors, which then enables us to separate the Teukolsky equation and compute the Newman-Penrose scalar analytically to third order in our expansion parameter. We obtain generic expressions for the mass and angular momentum flux through the perturbed black hole horizon, as well as the rate of change of the horizon surface area, in terms of certain invariants constructed from the electric and magnetic tidal tensors. We conclude by applying these results to the two scenarios described above.