Disc-oscillation resonance and neutron star QPOs: 3:2 epicyclic orbital model

The high-frequency quasi-periodic oscillations (HF QPOs) that appear in the X-ray fluxes of low-mass X-ray binaries remain an unexplained phenomenon. Among other ideas, it has been suggested that a non-linear resonance between two oscillation modes in an accretion disc orbiting either a black hole o...

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Published in:Astronomy and astrophysics (Berlin) 2010-11, Vol.522, p.A72
Main Authors: Urbanec, M., Török, G., Šrámková, E., Čech, P., Stuchlík, Z., Bakala, P.
Format: Article
Language:English
Subjects:
Astronomy
Black holes (astronomy)
Earth, ocean, space
Eigenfrequencies
Equations of state
Exact sciences and technology
Modulation
Neutron stars
Orbitals
Stars
stars: neutron
X-rays
X-rays: binaries
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Summary:The high-frequency quasi-periodic oscillations (HF QPOs) that appear in the X-ray fluxes of low-mass X-ray binaries remain an unexplained phenomenon. Among other ideas, it has been suggested that a non-linear resonance between two oscillation modes in an accretion disc orbiting either a black hole or a neutron star plays a role in exciting the observed modulation. Several possible resonances have been discussed. A particular model assumes resonances in which the disc-oscillation modes have the eigenfrequencies equal to the radial and vertical epicyclic frequencies of geodesic orbital motion. This model has been discussed for black hole microquasar sources as well as for a group of neutron star sources. Assuming several neutron (strange) star equations of state and Hartle-Thorne geometry of rotating stars, we briefly compare the frequencies expected from the model to those observed. Our comparison implies that the inferred neutron star radius RNS is larger than the related radius of the marginally stable circular orbit rms for nuclear matter equations of state and spin frequencies up to 800 Hz. For the same range of spin and a strange star (MIT) equation of state, the inferrred radius is RNS ~ rms. The “Paczyński modulation” mechanism considered within the model requires that RNS 
ISSN:0004-6361
1432-0746
DOI:10.1051/0004-6361/201014731