Flip-flopping binary black holes

TL;DR

This paper investigates the complex spin dynamics of binary black holes through long-term numerical simulations, revealing a flip-flop behavior of spins that impacts astrophysical observations and models.

Contribution

It provides the first detailed numerical simulation of long-term spin flip-flop cycles in binary black holes, supported by analytical expressions for flip-flop angles and frequencies.

Findings

Black hole spins can undergo complete flip-flops during inspiral.

Numerical simulations match Post-Newtonian predictions of spin dynamics.

Flip-flop behavior influences spin evolution and observational signatures.

Abstract

We study binary spinning black holes to display the long term individual spin dynamics. We perform a full numerical simulation starting at an initial proper separation of $d\approx25M$ between equal mass holes and evolve them down to merger for nearly 48 orbits, 3 precession cycles, and half of a flip-flop cycle. The simulation lasts for $t=20000M$ and displays a total change in the orientation of the spin of one of the black holes from initially aligned with the orbital angular momentum to a complete anti-alignment after half of a flip-flop cycle. We compare this evolution with an integration of the 3.5 Post-Newtonian equations of motion and spin evolution to show that this process continuously flip-flops the spin during the lifetime of the binary until merger. We also provide lower order analytic expressions for the maximum flip-flop angle and frequency. We discuss the effects this dynamics may have on spin growth in accreting binaries and on the observational consequences for galactic and supermassive binary black holes.