Distinguishing binary black hole precessional morphologies with gravitational wave observations*

The precessional motion of binary black holes can be classified into one of three morphologies, based on the evolution of the angle between the components of the spins in the orbital plane: Circulating, librating around 0, and librating around pi. All binaries are circulating at infinite separation, but they can transition to a librating morphology as they approach merger. These different morphologies close to merger can be related to the binary's formation channel and are imprinted in the binary's gravitational wave signal. We consider the prospects for distinguishing between the different morphologies close to merger using gravitational waves in the Advanced LIGO/Advanced Virgo network with their plus-era sensitivities. Here we consider fiducial high- and low-mass binaries and use Bayesian model selection to determine which morphology is preferred. We have implemented a fast calculation of the morphology, allowing us to restrict to a specific morphology in our stochastic sampling. We find that in the higher SNR cases with well-measured spin angles (most cases with larger spins) we have overwhelming evidence for the true morphology compared to at least one alternative morphology, and even find moderate evidence in many lower-SNR cases. Moreover, we are better able to distinguish the morphologies at the chosen reference point when the binary is not close to a morphology transition. We find that there is no preference for any morphology for the likely precessing binary black hole signal GW200129_065458.