2008ApJ...679.1413F

We propose that high-frequency quasi-periodic oscillations (HFQPOs) can be produced from randomly formed X-ray bursts (flashes) by plasma interior to the ergosphere of a rapidly rotating black hole. We show by direct computation of their orbits that the photons comprising the observed X-ray light curves, if due to a multitude of such flashes, are significantly affected by the black hole's dragging of inertial frames; the photons of each such burst arrive to an observer at infinity in multiple (double or triple), distinct ``bunches'' separated by a roughly constant time lag of Δt<sub>lag</sub>/M≃14, regardless of the bursts' azimuthal position. We argue that every other such ``bunch'' represents photons that follow trajectories with an additional orbit around the black hole at the photon circular orbit radius (a photon ``echo''). The presence of this constant lag in the response function of the system leads to a QPO feature in its power density spectra, even though the corresponding light curve consists of a totally stochastic signal. This effect is by and large due to the black hole spin and is shown to gradually diminish as the spin parameter a decreases or the radial position of the burst moves outside the static limit surface (ergosphere). Our calculations indicate that for a black hole with Kerr parameter of a/M=0.99 and mass of M=10 M_☉, the QPO is expected at a frequency of ν_QPO∼1.3-1.4 kHz. We discuss the plausibility and observational implications of our model results, as well as its limitations.