Off-equatorial orbits in strong gravitational fields near compact objects

TL;DR

This paper investigates the existence and stability of off-equatorial charged particle orbits near compact objects like black holes and stars, considering electromagnetic and gravitational interactions, and finds such orbits are possible under certain conditions.

Contribution

It demonstrates the conditions under which stable off-equatorial orbits of charged particles can exist near compact objects with magnetic fields, extending previous theoretical models.

Findings

Stable halo orbits exist around magnetized compact stars.

Such orbits are absent outside black hole horizons in Kerr-Newman solutions.

Off-equatorial orbits are either hidden or require naked singularities.

Abstract

Near a black hole or an ultracompact star, motion of particles is governed by strong gravitational field. Electrically charged particles feel also electromagnetic force arising due to currents inside the star or plasma circling around. We study a possibility that the interplay between gravitational and electromagnetic action may allow for stable, energetically bound off-equatorial motion of charged particles. This would represent well-known generalized Stormer's 'halo' orbits, which have been discussed in connection with the motion of dust grains in planetary magnetospheres. We demonstrate that such orbits exist and can be astrophysically relevant when a compact star or a black hole is endowed with a dipole-type magnetic field. In the case of Kerr-Newman solution, numerical analysis shows that the mutually connected gravitational and electromagnetic fields do not allow existence of stable halo orbits above the outer horizon of black holes. Such orbits are either hidden under the inner black-hole horizon, or they require the presence of a naked singularity.