Dynamical von Zeipel-Lidov-Kozai Oscillations of a Binary on a Spherical Orbit around a Rotating Supermassive Black Hole

Abstract

We study the dynamics of a binary system orbiting a rotating supermassive black hole (SMBH). Using Fermi-Walker transport, we construct a local inertial reference frame in the Kerr spacetime and set up a Newtonian binary system. Assuming the binary moves on a spherical orbit with constant radius around the black hole, we derive the equations of motion governing its dynamics. We focus on von Zeipel-Lidov-Kozai (vZLK) oscillations, which arise when the binary is compact and its initial inclination exceeds a critical angle. In our previous work on a circular orbit in the equatorial plane, we found that for hard binary systems, these oscillations in eccentricity and inclination follow a regular pattern, whereas in soft binaries, they exhibit chaotic behavior with irregular periods and amplitudes, yet remain stable. In this study, we extend our analysis to a spherical orbit in the Kerr background. The libration of the binary's orbit in the latitudinal direction affects the vZLK oscillations: as the libration angle increases, the oscillation period shortens, and the maximum eccentricity grows, particularly when the oscillations become chaotic. Notably, when the binary is sufficiently soft yet remains stable, the oscillation period is reduced to the dynamical timescale rather than the secular timescale. This effect arises due to the interaction between the SMBH spin and the binary's angular momentum. As the Kerr rotation parameter increases or the radius of the spherical orbit decreases, both the enhancement of maximum eccentricity and the reduction in oscillation period become more pronounced.