Quasi-circular Orbits for Spinning Binary Black Holes

TL;DR

This paper investigates the properties of quasi-circular orbits and the innermost stable circular orbit (ISCO) in spinning binary black hole systems using an effective potential approach, highlighting the influence of spin and limitations of the method.

Contribution

It introduces an effective potential method to analyze spinning binary black holes and characterizes how spin affects the ISCO and orbit stability.

Findings

ISCO position depends strongly on black hole spins

Sequences match post-Newtonian expansions at large separations

Effective potential method fails at close separations due to horizon formation

Abstract

Using an effective potential method we examine binary black holes where the individual holes carry spin. We trace out sequences of quasi-circular orbits and locate the innermost stable circular orbit as a function of spin. At large separations, the sequences of quasi-circular orbits match well with post-Newtonian expansions, although a clear signature of the simplifying assumption of conformal flatness is seen. The position of the ISCO is found to be strongly dependent on the magnitude of the spin on each black hole. At close separations of the holes, the effective potential method breaks down. In all cases where an ISCO could be determined, we found that an apparent horizon encompassing both holes forms for separations well inside the ISCO. Nevertheless, we argue that the formation of a common horizon is still associated with the breakdown of the effective potential method.