Up-down binaries are unstable and we want to know

The relativistic spin-precession equations admit equilibrium solutions corresponding to binaries with (anti) aligned spins. Among these, binaries in the up-down configuration, where the spin of the heavier (lighter) black hole is co- (counter-) aligned with the orbital angular momentum, are unstable to small perturbations of the spin directions. The occurrence of the up-down instability might lead to binaries that form with aligned spins but are detected with precessing spins. Crucially, unstable up-down sources do not disperse in the available parameter space at the end of their evolution but evolve toward a specific endpoint. This provides a clean observational prediction. We simulate unstable up-down binaries and investigate their distinguishability with current LIGO/Virgo interferometers. We also analyze the events of the third gravitational-wave transient catalog (GWTC-3) and search for promising candidates that could have encountered the up-down instability during their evolution.