Relativistic Accretion Disks: Low-Frequency Modes and Frame Dragging

A recently developed formalism is employed to study modes of pulsation of relativistic accretion disks around Kerr black holes. In the present study the disk's self-gravity is neglected, and attention is focused on the existence of modes with very low frequencies and their possible connection with the QPO phenomena exhibited by certain black hole candidates. A by-product is the first available comparison of relativistic and pseudo-Newtonian results. The analyses reveal that in many cases thin relativistic disks exhibit warped tilt/ precession modes with frequencies of the order of those associated with QPO phenomena. In the limit of a nonrotating black hole, the modes are all retrograde relative to the disk, with the relativistic frequencies smaller than the corresponding pseudo-Newtonian frequencies by about a factor of 2, due to redshift effects. This correspondence breaks down completely, however, as the black hole is spun up in the direction of disk rotation, due to effects associated with the relativistic phenomenon of the dragging of inertial frames. As the black hole is spun up, frame dragging influences the modes to an unprecedented degree, successively pulling modes through zero frequency so that they become prograde. For disk temperatures appropriate to X-ray emission, frame-dragging effects are dominant even for very small amounts of rotation. These effects allow an explanation in terms of a common mechanism for the full range of observed QPO frequencies (∼0.01-10 Hz).