Publication date: Dec 2007
Abstract:
High-frequency QPOs in neutron-star binary systems could be explained by
models based on parametric or forced resonance between oscillation modes
of the accretion disc around the neutron star (with frequencies related
to the Keplerian and epicyclic frequencies of the disc) and the relative
rotational motion between the disc material and either the neutron star
or the binary companion. Using Newtonian theory, we discuss the
possibility of forced resonant phenomena being excited by means of
gravitational perturbations coming from surface features on the neutron
star and from the companion star. For each potential perturbation
source, we have determined the Fourier decomposition of the
gravitational perturbing forces acting on disc elements in the radial
and vertical directions. The analysis presented for the binary companion
can be applied also to black hole systems but the surface features would
not be present in that case. The oscillations induced by the binary
partner are of a different character from those which would be induced
by a mountain or by the accretion columns. In the case of symmetric
accretion columns, the excitation frequency in the radial direction is
twice that in the vertical direction and higher order modes could be
relevant for parts of the disc very close to the neutron star. In this
inner region, the influence of the accretion columns is greater than
that of the binary companion for producing forced oscillations in both
the radial and vertical directions; in the intermediate part, the
vertical oscillations are induced by the accretion columns while the
radial oscillations are excited by the binary partner (if appropriate
conditions are fulfilled); sufficiently far from the neutron star, the
binary companion has the greater effect. The limits on the magnitude of
neutron star deformations given by the possibility to excite observable
QPOs coincide with restrictions coming from (non-)observations of
gravitational waves by LIGO.
Authors:
Stuchlík, Zdeněk; Konar, Sushan; Miller, John C.; Hledík, Stanislav;