Spin-orbit interactions in black-hole binaries

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 quasicircular configurations, one with initially nonspinning 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 nonspinning 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 nonboosted 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.