Induced Spins from Hyperbolic and Near-Hyperbolic Encounters of Initially Nonspinning Black Holes

When two nonspinning, equal-mass black holes pass by one another on hyperbolic or near-hyperbolic trajectories, we would expect their tidal interaction to induce spins on each black hole. However, neither perturbative (e.g. post-Newtonian) treatments nor numerical relativity simulations to date have explored tidally induced spins from such non-merging encounters. We have performed a number of numerical relativity simulations to study this potential effect. The basic set-up for the simulations involves two equal-mass, nonspinning black holes (Bowen-York initial data) at 100M initial separation. We vary two parameters: the initial boost and shooting angle 𝜃 of the black holes. As the shooting angle 𝜃 decreases toward the bound/unbound limit (corresponding to marginally bound zoom-whirl orbits), tidally induced spin-ups increase exponentially. We find that initial boosts of 0.265c lead to interactions that spin up each black hole to a dimensionless spin parameter of at most 0.0197 (corresponding to the bound/unbound limit). With initial boosts of 0.488c, this grows to 0.199. Based on these results, we anticipate that much higher spin-ups will be possible with larger initial boosts.