A Model for Quasi-periodic Oscillations in Cataclysmic VariablesBased on Boundary Layer Oscillations

Abstract

We have performed a linearized perturbation analysis of the disk and boundary layer (BL) in a cataclysmic variable. In the BL, we find a triplet of large-scale, torsional oscillation modes consisting primarily of perturbations of the azimuthal velocity, together with a singlet mode consisting primarily of a pressure perturbation. Two of these, which we call the fast torsional modes, have rise times short enough to provide significant amplification of a perturbation carried inward through the BL with the local drift velocity. These are gravity modes, modified by the large shear and radial acceleration in the BL. The two fast torsional modes have frequencies close enough in magnitude to produce beating; the underlying oscillations have very short periods, ~1 s, which may be observable. The resulting beat frequency ranges from the local orbital rotation period, 2π/Ω, up to an order of magnitude larger than that, i.e., ~20 to (2-3)×100 s, through the region of the boundary layer in which the effective temperature reaches a maximum. The beat oscillations between the fast torsional modes have attributes similar to the quasi-periodic oscillations (QPOs) observed in some cataclysmic variables. We suggest that these beat oscillations represent a class of QPOs, and we suggest some tests of this hypothesis.