The anomalous orbital velocity effect and high frequency quasi-periodic oscillations in accreting black holes

The recently discovered anomalous orbital velocity effect, which implies a reduced velocity with decreasing radius for an orbiting test particle, is present in extreme Kerr black holes with spin > 0.9953 and radii between 1.05 and 1.78 gravitational radii. In this region a 3:1 resonance between the vertical and the radial epicyclic frequencies occurs, which may lead to quasi-peridoc light variations with two periods differing by a factor of 3. Such variations may have been observed from the Galactic Center black hole Sgr A*. At the same spin but at a larger radius a 3:2 resonance between the two epicyclic frequencies is found as well. Such 3:2 frequencies have been observed in four galactic microquasars. In either case, i.e. the 3:1 or the 3:2 resonance, the black hole mass determined from this model agrees very well with the dynamical mass for Sgr A* and each of the four microquasars. Meanwhile the microquasar GRS 1915+105 has been classified by its X-ray spectrum as an extreme Kerr black hole with a spin > 0.98 [1], which agrees quite nicely with our prediction for the spin, i.e. a 0.996. Model and observational data are presented and discussed.