Abstract

Semi-periodic variability has been observed for years in emission from black hole candidates, primarily by the Rossi X-Ray Timing Explorer, with `quasi-periodic oscillations’ (QPOs) appearing as coherent peaks in the power density spectrum as frequencies of ~0.1-450 Hz. The QPOs with higher frequencies of 40-450Hz (HFQPOs) attract particular interest because their frequencies roughly match those expected for matter orbiting near the innermost stable circular orbit (ISCO), and remain stable to variations in luminosity. These characteristics suggest that HFQP Os might offer a probe into the intrinsic properties of the central black hole, specifically the spin angular momentum. One explanation for HFQP Os is the trapping and amplification of global `diskoseismic’ oscillations in the inner regions of the surrounding accretion disk, in particular the so-called `gravito-inertial’ waves, or r-modes. In a purely hydrodynamic context, r-modes can be confined by relativistic effects within a resonant cavity distinct from the ISCO , and can grow in amplitude through couplings with warps and eccentricities. However, recent work has called into question the survival of trapped inertial waves in the presence of magnetohydrodynamic turbulence, and the very existence of the self-trapping region in an accretion disk threaded by large-scale magnetic fields. I summarise semi-analytical and numerical work aimed at addressing these questions of existence and persistence more fully.