Probing strong-gravity chaos in rotating Kerr–Bertotti–Robinson black holes

We study the dynamics of test particles in the spacetime of a rotating and axially symmetric Kerr–Bertotti–Robinson (KBR) black hole and examine the dependence of orbital motion on the model parameters. The stability of null circular geodesics is analyzed using the largest Lyapunov exponent, providing a quantitative criterion for orbital instability. Chaotic behavior is further investigated through Poincaré sections, which reveal the global structure of phase space and the transition from regular to chaotic motion. The divergence of nearby trajectories is quantified using both the largest Lyapunov exponent and the Fast Lyapunov Indicator, offering complementary diagnostics of dynamical stability. Furthermore, the Kolmogorov–Sinai (KS) entropy is also employed to provide a quantitative characterization of chaos in the dynamical system. Our results demonstrate that the presence of an external magnetic field significantly modifies the geodesic dynamics compared to the Kerr spacetime, leading to qualitative differences in orbital stability and chaotic features. These findings underscore the crucial role of electromagnetic fields in shaping the nonlinear dynamics of particles around rotating black holes.