Spin-orbit interactions in black-hole binaries

TL;DR

This study uses numerical simulations to analyze how spin and orbital angular momentum are exchanged during the final stages of black-hole binary mergers, revealing weak spin-orbit coupling and angular momentum transfer effects.

Contribution

It provides the first detailed numerical analysis of spin-orbit interactions and angular momentum transfer during the late inspiral and merger of black-hole binaries.

Findings

Spin increase during last orbits is minimal, about 0.012 and 0.006.

Spin-orbit coupling is too weak to cause tidal locking.

Angular momentum loss correlates with initial spins and orbital dynamics.

Abstract

We perform numerical simulations of black-hole binaries to study the exchange of spin and orbital angular momentum during the last, highly nonlinear, stages of the coalescence process. To calculate the transfer of angular momentum from orbital to spin, we start with two quasi-circular configurations, one with initially non-spinning black holes, the other with corotating black holes. In both cases the binaries complete almost two orbits before merging. We find that, during these last orbits, the specific spin (a/m) of each horizon increases by only 0.012 for the initially non-spinning configuration, and by only 0.006 for the initially corotating configuration. By contrast, the corotation value for the specific spin should increase from 0.1 at the initial proper separation of 10M to 0.33 when the proper separation is 5M. Thus the spin-orbit coupling is far too weak to tidally lock the binary to a corotating state during the late-inspiral phase. We also study the converse transfer from spin into orbital motion. In this case, we start the simulations with parallel, highly-spinning non-boosted black holes. As the collision proceeds, the system acquires a non-head-on orbital motion, due to spin-orbit coupling, that leads to the radiation of angular momentum. We are able to accurately measure the energy and angular momentum losses and model their dependence on the initial spins.