Probing strong-gravity chaos and stability of circular photon orbits around the Kerr–Sen black hole

Abstract This study presents a detailed investigation of the dynamics of massless test particles (photons) to analyze the stability of null circular geodesics around the Kerr–Sen black hole (KSBH) spacetime. KSBH is a new solution arising in the low-energy effective field theory of heterotic string theory. The analysis proceeds by obtaining the radial effective potential, which serves as the foundation for calculating the Lyapunov exponent, a central quantity used to describe the stability or instability of geodesic motion in dynamical systems. The study employs the Lyapunov exponent approach to determine the stability or instability of circular orbits under varying values of the charge parameter Q and Kerr parameter a . Additionally, the analytical expressions for the radius of circular photon orbits and the corresponding impact parameter, defined as the ratio of conserved angular momentum to energy are also derived explicitly. To further analyze the orbital behavior, techniques such as Poincaré maps, the Lyapunov Exponent, and FLI are utilized to distinguish between regular and chaotic trajectories. The sign and magnitude of the Lyapunov exponent provide a rigorous criterion to distinguish between stable and unstable circular orbits. These results contribute to a deeper understanding of the geodesic structure and perturbative stability in string-inspired rotating charged black hole backgrounds. The findings are also compared with those of the KBH case to highlight the impact of the BH charge on the stability of null geodesics. This study provides deeper insights into the dynamical properties of null geodesics in charged rotating BH spacetimes in GR and alternative theory of gravity.