Charged particles orbiting black hole mimickers with dipole magnetosphere

Abstract We investigate the dynamics of charged test particles in the vicinity of magnetized black hole mimickers, such as gravastars, wormholes, and ultracompact Buchdahl stars, modeled by a Schwarzschild spacetime with an external dipole magnetic field originating inside the mimicker. The interaction between gravity and electromagnetism is characterized by a single dimensionless “magnetic” parameter b that controls the relative strength and orientation of the Lorentz force. We provide a complete classification of equatorial and off-equatorial circular orbits across all regimes of the magnetic parameter, with particular emphasis on the strong-field region below the photon sphere. We show that stable off-equatorial circular orbits arise only under magnetic repulsion and originate at the radii where equatorial orbits become marginally unstable to vertical perturbations. Remarkably, such off-equatorial orbits can remain stable even inside the photon sphere, preventing direct particle infall as in geodesic motion. We further analyze epicyclic oscillations around stable orbits and discuss their relevance for high-frequency quasi-periodic oscillations observed in many black hole systems. Outside the regime of regular oscillations, we study chaotic charged particle motion in belts concentrated around off-equatorial circular orbits, including the effects of radiative back-reaction, which may be relevant to coronae around accretion disks. Finally, we study the ionization of initially Keplerian disks and demonstrate qualitatively different dynamical outcomes for magnetically repelled and attracted particles. We present possible observational signatures of the considered black hole mimickers, most notably the existence of a high-energy particle repulsive barrier located under the photon sphere.