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

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.

10 pages, 3 figures, accepted for publication in ApJ

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Cited by in corpus (12)

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• Stability of the Magnetopause of Disk-Accreting Rotating Stars
• Constraining the size of the Comptonizing medium by modeling the energy dependent time-lags of kHz QPOs of Neutron star system
• High-frequency variability in neutron-star low-mass X-ray binaries
• The amplitude of the kilohertz quasi-periodic oscillations in 4U 1636$-$53 in the frequency-energy space
• On the high-frequency quasi-periodic oscillations from black holes
• Kilohertz quasi-periodic oscillations from neutron star spreading layers
• Constraints on the disc-magnetosphere interaction in accreting pulsar 4U 1626--67
• Parallel tracks as quasi-steady states for the magnetic boundary layers in neutron-star low-mass X-ray binaries
• A new correlation with lower kilohertz quasi-periodic oscillation frequency in the ensemble of low-mass X-ray binaries
• On kHz oscillations and characteristic frequencies of accreting magnetospheres
• QPO frequency derivative - frequency correlation indicates non-Keplerian boundary layer with a maximum in rotation rate