Modeling horizon absorption in spinning binary black holes using effective worldline theory

The mass and spin of black holes (BHs) in binary systems may change due tothe infall of gravitational-wave (GW) energy down the horizons. For spinningBHs, this effect enters at 2.5 post-Newtonian (PN) order relative to theleading-order energy flux at infinity. There is currently a discrepancy in theliterature in the expressions of these horizon fluxes in the test-body limit at4PN order (relative 1.5PN order). Here, we model the horizon absorption astidal heating in an effective worldline theory of a spinning particle equippedwith tidally-induced quadrupole and octupole moments. We match the tidalresponse to analytic solutions of the Teukolsky equation in a scatteringscenario, and obtain general formulae for the evolution of mass and spin. Wethen specialize to the case of aligned-spin--quasi-circular binaries, obtainingthe corresponding contributions to the GW phasing through 4PN order.Importantly, we find that the number of GW cycles due to horizon fluxes withmasses observed by LIGO-Virgo-KAGRA detectors is about 2-3 orders of magnitudesmaller than the other contributions to the phasing at the same PN order.Furthermore, in the test-body limit, we find full agreement with resultsobtained earlier from BH perturbation theory, with a small mass in anequatorial circular orbit treated as a source perturbing the Kerr metric. Thus,we weigh in on one side of the previous discrepancy.