Aschenbach effect for spinning particles in Kerr spacetime

The orbital velocity profile of circular timelike geodesics in the equatorial plane of a Kerr black hole has a nonmonotonic radial behavior, provided that the spin parameter a of the black hole is bigger than a certain critical value a(c) approximate to 0.9953M. Here the orbital velocity is measured with respect to the locally nonrotating frame, and the nonmonotonic behavior, which is known as the Aschenbach effect, occurs only for corotating orbits. Using the Mathisson-Papapetrou-Dixon equations for a massive spinning particle, we investigate the Aschenbach effect for test particles with spin. In addition to the black-hole spin, the absolute value of the particle's spin and its orientation (parallel or antiparallel to the black-hole spin) also play an important role for the Aschenbach effect. We determine the critical value a(c) of the spin parameter of the Kerr black hole where the Aschenbach effect sets in as a function of the spin of the probe. We consider not only black holes (a(2) <= M-2) but also naked singularities (a(2) > M-2). Whereas for spinless (geodesic) particles the orbital velocity is always monotonically decreasing if the motion is counterrotating, we find that for spinning particles in counterrotating motion with antiparallel spin around a naked singularity the orbital velocity is increasing on a certain radius interval.