In at least two black hole binary systems, the Rossi X-Ray Timing Explorer
has detected high frequency quasi-periodic oscillations (HFQPOs) with a 2:3
frequency commensurability. We propose a simple hot spot model to explain the
positions, amplitudes, and widths of the HFQPO peaks. Using the exact geodesic
equations for the Kerr metric, we calculate the trajectories of massive test
particles, which are treated as isotropic, monochromatic emitters in their rest
frames. By varying the hot spot parameters, we are able to explain the
different features observed in ``Type A'' and ``Type B'' QPOs from XTE
J1550-564. In the context of this model, the observed power spectra allow us to
infer values for the black hole mass and angular momentum, and also constrain
the parameters of the model.Comment: 4 pages, 2 figures, to be published in X-Ray Timing 2003: Rossi and
Beyond, ed. P. Kaaret, F. K. Lamb, & J. H. Swank (Melville, NY: American
Institute of Physics
