Innermost Stable Circular Orbit of a Spinning Particle in Kerr Spacetime

TL;DR

This paper investigates the stability of spinning test particles in Kerr spacetime, revealing that spin affects the innermost stable circular orbit (ISCO) and its associated frequency, especially for large spins.

Contribution

It demonstrates that spin induces instability in certain orbital directions and alters the ISCO's radius and frequency in Kerr spacetime.

Findings

Some circular orbits become unstable perpendicular to the equatorial plane.

The ISCO appears before the effective potential minimum disappears for large spins.

Spin significantly changes the radius and frequency of the last stable orbit.

Abstract

We study stability of a circular orbit of a spinning test particle in a Kerr spacetime. We find that some of the circular orbits become unstable in the direction perpendicular to the equatorial plane, although the orbits are still stable in the radial direction. Then for the large spin case ($S < \sim O(1)), the innermost stable circular orbit (ISCO) appears before the minimum of the effective potential in the equatorial plane disappears. This changes the radius of ISCO and then the frequency of the last circular orbit.