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 non-monotonic radial behavior, provided that the spin parameter $a$ of the black hole is bigger than a certain critical value $a_c \approx 0.9953 M$. Here the orbital velocity is measured with respect to the Locally Non-Rotating Frame (LNRF), and the non-monotonic behavior, which is known as the Aschenbach effect, occurs only for co-rotating 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 anti-parallel 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 ($a^2 \le M^2$) but also naked singularities ($a^2>M^2$). Whereas for spinless (geodesic) particles the orbital velocity is always monotonically decreasing if the motion is counter-rotating, we find that for spinning particles in counter-rotating motion with anti-parallel spin around a naked singularity the orbital velocity is increasing on a certain radius interval.