## Remnant black hole properties

When two black holes (BHs) merge, the end state is believed to be a Kerr black hole, characterized by only its mass, angular momentum, and linear momentum. All other degrees of freedom, a.k.a. hair, are radiated away as gravitational waves. From far away, this process can be viewed as a scattering problem: The initial state is defined by spins and masses of the two BHs and the final state is defined by the mass, spin and recoil velocity of the remnant BH.

Modeling the remnant properties of binary black holes is of great astrophysical interest. It is a crucial element in tests of general relativity (GR), particularly testing the Kerr hypothesis, no hair theorem, consistency of GR in predicting the inspiral and ringdown, and so on. The remnant properties are also important in modeling the ringdown part of the gravitational wave signal, since the quasi-normal mode frequencies are entirely determined by the remnant. Because of the highly dynamical nature of the merger, prediction of the remnant properties is only possible through numerical simulations.

Since numerical simulations are very expensive, several fits have been developed over the years for the remnant mass, spin and recoil kick as functions of the component parameters. However, most of these have only been calibrated against simulations where the component spins are aligned to the orbital angular momentum. When the spins are misaligned, they couple with the orbital angular momentum, and each other and cause the orbital plane to precess. Therefore, precessing BBH are much harder to model, but they also have much richer physics!

In this work we construct fits for the remnant properties of precessing BBH by directly training our model against hundreds of precessing numerical simulations. We show more than an order magnitude improvement in the accuracy of predicting remnant properties. These results are presented in 1809.09125, and our models are available through a public Python package surfinBH. Also, our paper was covered by a Caltech press release!

Here's a visualization of the inspiral and merger of a precessing BBH system. After the BHs merge, you can see the final remnant BH. At the end, the movie is sped up and you can see the remnant BH fly away with the recoil velocity. See: binaryBHexp for more details about this animation.