Publication date: Nov 2010
Abstract:
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 <
rms. However, we find this condition to be fulfilled only for
the strange matter equation of state, masses below 1 M⊙,
and spin frequencies above 800 Hz. This result most likely falsifies the
postulation of the neutron star 3:2 resonant eigenfrequencies being
equal to the frequencies of geodesic radial and vertical epicyclic
modes. We suggest that the 3:2 epicyclic modes could stay among the
possible choices only if a fairly non-geodesic accretion flow is
assumed, or if a different modulation mechanism operates.
Authors:
Urbanec, M.; Török, G.; Šrámková, E.; Čech, P.; Stuchlík, Z.; Bakala, P.;