Geodesic motion in a stationary dihole spacetime

TL;DR

This paper analyzes geodesic motion around a binary system of counter-rotating Kerr-Newman black holes, revealing conditions for bounded orbits, effects of crossing the strut, and the innermost stable orbit.

Contribution

It provides a detailed analysis of geodesics in a dihole spacetime, including photon and particle trajectories, and explores the innermost stable circular orbit in this specific binary black hole configuration.

Findings

Bounded and unbounded orbits are possible.

Particles can cross between black holes only if angular momentum is zero.

The innermost stable circular orbit depends on black hole parameters.

Abstract

The knowledge of the properties of the different exact solutions modeling binary systems, is a necessary step towards the classification of physically suitable solutions and its corresponding limits of applicability. In the present paper, we perform an analysis of the geodesics around two counter--rotating Kerr--Newman black holes endowed with opposite electric charges, which achieve equilibrium by means of a strut between their constituents. We find that bounded and unbounded orbits are possible. However, test particles may cross between the black holes only if their angular momentum equals zero, otherwise, there exist a repulsive potential, which prohibits such orbits. Two important aspects are pointed out for these trajectories: ({\it i}) the motion of photons is affected once crossing the strut; and ({\it ii}) massive particles exhibit oscillatory motion, as a first analog of the Sitnikov problem in general relativity. The radius of the innermost stable circular orbit as a function of the physical parameters of the black holes is also investigated.