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

Abstract

In this work, we investigate bound periodic orbits of timelike particles in the spacetime of dyonic black holes arising from quasi-topological electromagnetic theory. By varying the coupling parameter $\alpha_1$, the corresponding black hole solutions exhibit diverse horizon structures, including naked singularities and black holes with one to four horizons. We find that for sufficiently small $\alpha_1$, the metric function $f(r)$ becomes non-monotonic outside the event horizon in spacetimes with one or two horizons, while in all other cases, $f(r)$ remains strictly monotonic. In the non-monotonic regime, the radial effective potential develops a double-barrier structure, allowing the emergence of multiple marginally bound orbits and multiple branches of periodic orbits associated with the same rational number $l$. Although differing in radial structure, these orbit branches are topologically equivalent. Remarkably, when the outer potential barrier exceeds unity, bound orbits with energy $E>1$ become possible, in addition to the standard $E<1$ branches. When the peak reaches $E=1$, up to three distinct bound orbit branches may coexist. We also identify a novel eccentricity behavior, the innermost branch becomes increasingly circular with increasing energy or angular momentum, while outer branches exhibit greater eccentricity and a larger apastron-periastron separation. These features, absent in previous studies, are unique signatures of non-monotonic metric functions. In contrast, monotonic cases yield a single-well potential, a unique marginally bound orbit, and a single periodic orbit branch per $q$, consistent with earlier findings. Our results highlight the critical role of the metric function's shape in determining the orbital structure around dyonic black holes.