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;