A new type of multi-branch periodic orbits in dyonic black holes

Abstract

We investigate bound timelike periodic orbits in dyonic black hole spacetimes arising from quasi-topological electromagnetism. By varying the coupling parameter $\alpha_1$, we show that the exterior monotonicity of the metric function, rather than the number of horizons, controls the topology of the radial effective potential, which can exhibit either a single well or multiple wells separated by potential barriers. When $f(r)$ is non-monotonic outside the event horizon, the effective potential develops multiple wells, leading to multiple MBO branches and several coexisting periodic-orbit branches with the same rational number $q$. These branches are topologically equivalent but geometrically distinct, because they correspond to different energies or angular momenta, leading to different radial extents and eccentricities. In particular, bound periodic orbits with $E>1$ can occur, and up to three branches may coexist. We also find an inverted radial response: the innermost branch becomes more circular as the energy or angular momentum increases, whereas the outer branches become more eccentric. By contrast, when $f(r)$ is monotonic outside the event horizon, the effective potential has a single well and only one periodic orbit branch exists, even for black holes with multiple horizons. Our results identify metric non-monotonicity as the geometric origin of multi-branch periodic motion and suggest a timelike counterpart to the multiple photon ring signatures of nonstandard black hole geometries.