The development and influence of global oscillation modes in simulated viscous black hole accretion disks

Sean M. O'Neill, University of Maryland, College Park

We examine the evolution and influence of viscosity-driven diskoseismic modes in simulated black hole accretion disks. Understanding the behavior of such oscillations will help us to evaluate their potential role in producing astronomically observed high-frequency quasi-periodic oscillations in accreting black hole systems.

Our simulated disks are geometrically-thin with a constant half-thickness of five percent the radius of the innermost stable circular orbit. A pseudo-Newtonian potential reproduces the relevant effects of general relativity, and an alpha-model viscosity achieves angular momentum transport and the coupling of orthogonal velocity components in an otherwise ideal hydrodynamic numerical treatment.

We find that our simulated viscous disks characteristically develop and maintain trapped global g-mode oscillations in a narrow range of frequencies just below the maximum radial epicyclic frequency. While the modes are driven in the inner portion of the disk, they generate waves that propagate at g-mode frequencies out to larger disk radii. This finding is contrasted with the results of magnetohydrodynamic simulations, in which such global oscillations are not easily identified. Such examples underscore a fundamental physical difference between accretion systems driven by the magneto-rotational instability and those for which alpha viscosity serves as a proxy for the physical processes that drive accretion.