Excitation of oscillations in accretion discs orbiting compact objects

Details Excitation of oscillations in accretion discs orbiting compact objects Excitation of oscillations in accretion discs orbiting compact objects

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007ragt.meet..293s&link_type=abstract

Proceedings of RAGtime 8/9: Workshops on black holes and neutron stars, 15-19/19-21 September, 2006/2007, Hradec nad Moravicí, O

Computer Science

1

Neutron Stars, Binary Systems, Qpos, Accretion Discs

Scientific paper

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.

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