Oscillations of the Inner Regions of Viscous Accretion Disks

Abstract

Although quasi-periodic oscillations (QPOs) have been discovered in different X-ray sources, their origin is still a matter of debate. Analytical studies of hydrodynamic accretion disks have shown three types of trapped global modes with properties that appear to agree with the observations. However, these studies take only linear effects into account and do not address the issues of mode excitation and decay. Moreover, observations suggest that resonances between modes play a crucial role. A systematic, numerical study of this problem is therefore needed. In this paper, we use a pseudo-spectral algorithm to perform a parameter study of the inner regions of hydrodynamic disks. By assuming alpha-viscosity, we show that steady state solutions rarely exist. The inner edges of the disks oscillate and excite axisymmetric waves. In addition, the flows inside the inner edges are sometimes unstable to non-axisymmetric perturbations. One-armed, or even two-armed, spirals are developed, which provides a plausible explanation for the high-frequency QPOs observed from accreting black holes. When the Reynolds numbers are above certain critical values, the inner disks go through some transient turbulent states characterized by strong trailing spirals; while large-scale leading spirals developed in the outer disks. We compared our numerical results with standard thin disk oscillation models. Although the non-axisymmetric features have their analytical counterparts, more careful study is needed to explain the axisymmetric oscillations.