Quasi-periodic oscillations as global hydrodynamic modes in the boundary layers of viscous accretion disks

TL;DR

This paper investigates how global hydrodynamic modes in the boundary layers of viscous accretion disks around neutron stars can explain observed quasi-periodic oscillations, linking mode frequencies to neutron star spin and disk parameters.

Contribution

It introduces a model of hydrodynamic modes in accretion disk boundary layers that accounts for QPO frequencies and their relation to neutron star spin.

Findings

Fastest growing mode frequencies are near the radial epicyclic frequency and its variants.

Mode frequency differences match observed kHz QPO separations and relate to neutron star spin.

Maximum growth occurs where the orbital frequency is highest.

Abstract

The observational characteristics of quasi-periodic oscillations (QPOs) from accreting neutron stars strongly indicate the oscillatory modes in the innermost regions of accretion disks as a likely source of the QPOs. The inner regions of accretion disks around neutron stars can harbor very high frequency modes related to the radial epicyclic frequency $\kappa $. The degeneracy of $\kappa $ with the orbital frequency $\Omega $ is removed in a non-Keplerian boundary or transition zone near the magnetopause between the disk and the compact object. We show, by analyzing the global hydrodynamic modes of long wavelength in the boundary layers of viscous accretion disks, that the fastest growing mode frequencies are associated with frequency bands around $\kappa $ and $\kappa \pm \Omega $. The maximum growth rates are achieved near the radius where the orbital frequency $\Omega $ is maximum. The global hydrodynamic parameters such as the surface density profile and the radial drift velocity determine which modes of free oscillations will grow at a given particular radius in the boundary layer. In accordance with the peak separation between kHz QPOs observed in neutron-star sources, the difference frequency between two consecutive bands of the fastest growing modes is always related to the spin frequency of the neutron star. This is a natural outcome of the boundary condition imposed by the rotating magnetosphere on the boundary region of the inner disk.