Accretion tori around rotating neutron stars – Paper II: Oscillations and precessions
arxiv(2024)
摘要
The four characteristic oscillation frequencies of accretion flows are, in
addition to the Keplerian orbital frequency, often discussed in the context of
the time variability of the black hole and neutron star (NS) low-mass X-ray
binaries (LMXBs). These are namely the frequencies of the axisymmetric radial
and vertical epicyclic oscillations, and the frequencies of non-axisymmetric
oscillations corresponding to the periastron (radial) and Lense-Thirring
(vertical) precessions. In this context, we investigate the effect of the
quadrupole moment of a slowly rotating NS and provide complete formulae for
calculating these oscillation and precession frequencies, as well as their
convenient approximations. Simple formulae corresponding to the geodesic limit
of a slender torus (and test particle motion) and the limit of a marginally
overflowing torus (torus exhibiting a critical cusp) are presented, and
furthermore, more general approximate formulae are included to allow
calculations for arbitrarily thick tori. We provide the Wolfram Mathematica
code used for our calculations together with C++ and PYTHON codes for
calculations of the frequencies. Our formulae can be used for various
calculations describing the astrophysical signatures of the NSs' superdense
matter equation of state. For instance, we demonstrate that, even for a given
fixed number of free parameters, a model accounting for fluid flow precession
better matches the frequencies of twin-peak quasiperiodic oscillations observed
in NS LMXBs than a model using geodesic precession.
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