Equatorial Circular Orbits, Epicyclic Frequencies, and Planar Accretion in Kerr–Sen Black Hole Spacetimes

In this paper, we examine the dynamics of test particles in the Kerr-Sen black hole (BH) spacetime and its properties. In this case, the study concentrates on equatorial circular orbits and their stability properties using the effective potential formalism approach. We derive conditions for stable circular motion and examine the influence of BH parameters on the observed dynamics properties of the orbital structure. Furthermore, we investigate small harmonic perturbations around circular orbits and calculate the associated oscillation frequencies measured by distant asymptotic observers. In this context, these frequencies provide a direct probe of the underlying geometry and deviation from the Kerr limit. We also perform a numerical analysis to illustrate the dependence of orbital stability and oscillatory behavior on the rotation and charge parameters of the Kerr–Sen solution. In addition, our general relativistic hydrodynamical simulations of two-dimensional equatorial planar accretion show that increasing the Kerr–Sen spacetime parameter q enhances the mass-accretion rate and modifies the local density and radial-velocity profiles near the black hole, with the strongest deviation from the Kerr case appearing for the largest value of q .