2017ApJ...846L..11L

We study the effect of an external companion on the orbital and spin evolution of merging black hole (BH) binaries. A sufficiently nearby and inclined companion can excite Lidov-Kozai (LK) eccentricity oscillations in the binary, thereby shortening its merger time. During such LK-enhanced orbital decay, the spin axis of the BH generally exhibits chaotic evolution, leading to a wide range (0°-180°) of the final spin-orbit misalignment angle from an initially aligned configuration. For systems that do not experience eccentricity excitation, only modest ( 20^○) spin-orbit misalignment can be produced, and we derive an analytic expression for the final misalignment using the principle of adiabatic invariance. The spin-orbit misalignment directly impacts the gravitational waveform and can be used to constrain the formation scenarios of BH binaries and dynamical influences of external companions.