2009ApJ...695.1199F

We continue our earlier studies of quasi-periodic oscillations (QPOs) in the power spectra of accreting, rapidly rotating black holes that originate from the geometric "light echoes" of X-ray flares occurring within the black hole ergosphere. Our present work extends our previous treatment to three-dimensional photon emission and orbits to allow for arbitrary latitudes in the positions of the distant observers and the X-ray sources in place of the mainly equatorial positions and photon orbits of the earlier consideration. Following the trajectories of a large number of photons, we calculate the response functions of a given geometry and use them to produce model light curves which we subsequently analyze to compute their power spectra and autocorrelation functions. In the case of an optically thin environment, relevant to advection-dominated accretion flows, we consistently find QPOs at frequencies on the order of ∼kHz for stellar-mass black hole candidates, while on the order of ∼mHz for typical active galactic nuclei (∼10⁷ M_☉) for a wide range of viewing angles (30°-80°) from X-ray sources predominantly concentrated toward the equator within the ergosphere. As in our previous treatment, here too, the QPO signal is produced by the frame-dragging of the photons by the rapidly rotating black hole, which results in photon "bunches" separated by constant time lags, the result of multiple photon orbits around the hole. Our model predicts for various source/observer configurations the robust presence of a new class of QPOs, which is inevitably generic to a curved spacetime structure in rotating black hole systems.