165 research outputs found
Magnetohydrodynamics in Superconducting-Superfluid Neutron Stars
MHD equations are presented for the mixture of superfluid neutrons,
superconducting protons, and normal electrons believed to exist in the outer
cores of neutron stars. The dissipative effects of electron viscosity and
mutual friction due to electron-vortex scattering are also included. It is
shown that Alfven waves are replaced by cyclotron- vortex waves that have not
been previously derived from MHD theory. The cyclotron- vortex waves are
analogous to Alfven waves with the tension due to the magnetic energy density
replaced by the vortex energy density. The equations are then put into a
simplified form useful for studying the effects of the interior magnetic field
on the dynamics. Of particular interest is the crust-core coupling time which
can be inferred from pulsar glitch observations. The hypothesis that
cyclotron-vortex waves play a significant role in the core spin-up during a
glitch is used to place limits on the interior magnetic field. The results are
compared with those of other studies.Comment: 11 pages, requires MN plain TeX macros, to appear in MNRAS. Sec. 6
was revised on 02/20/97 to state that stratification will greatly increase
the magnetic spin-up time; 09/09/97 version corrects minor typos onl
Using generalized PowerFlux methods to estimate the parameters of periodic gravitational waves
We investigate methods to estimate the parameters of the gravitational-wave
signal from a spinning neutron star using Fourier transformed segments of the
strain response from an interferometric detector. Estimating the parameters
from the power, we find generalizations of the PowerFlux method. Using
simulated elliptically polarized signals injected into Gaussian noise, we apply
the generalized methods to estimate the squared amplitudes of the plus and
cross polarizations (and, in the most general case, the polarization angle),
and test the relative detection efficiencies of the various methods.Comment: 8 pages, presented at Amalid7, Sydney, Australia (July 2007), fixed
minor typos and clarified discussion to match published CQG version; updated
reference
The r-modes in accreting neutron stars with magneto-viscous boundary layers
We explore the dynamics of the r-modes in accreting neutron stars in two
ways. First, we explore how dissipation in the magneto-viscous boundary layer
(MVBL) at the crust-core interface governs the damping of r-mode perturbations
in the fluid interior. Two models are considered: one assuming an
ordinary-fluid interior, the other taking the core to consist of superfluid
neutrons, type II superconducting protons, and normal electrons. We show,
within our approximations, that no solution to the magnetohydrodynamic
equations exists in the superfluid model when both the neutron and proton
vortices are pinned. However, if just one species of vortex is pinned, we can
find solutions. When the neutron vortices are pinned and the proton vortices
are unpinned there is much more dissipation than in the ordinary-fluid model,
unless the pinning is weak. When the proton vortices are pinned and the neutron
vortices are unpinned the dissipation is comparable or slightly less than that
for the ordinary-fluid model, even when the pinning is strong. We also find in
the superfluid model that relatively weak radial magnetic fields ~ 10^9 G (10^8
K / T)^2 greatly affect the MVBL, though the effects of mutual friction tend to
counteract the magnetic effects. Second, we evolve our two models in time,
accounting for accretion, and explore how the magnetic field strength, the
r-mode saturation amplitude, and the accretion rate affect the cyclic evolution
of these stars. If the r-modes control the spin cycles of accreting neutron
stars we find that magnetic fields can affect the clustering of the spin
frequencies of low mass x-ray binaries (LMXBs) and the fraction of these that
are currently emitting gravitational waves.Comment: 19 pages, 8 eps figures, RevTeX; corrected minor typos and added a
referenc
R-Modes in Superfluid Neutron Stars
The analogs of r-modes in superfluid neutron stars are studied here. These
modes, which are governed primarily by the Coriolis force, are identical to
their ordinary-fluid counterparts at the lowest order in the small
angular-velocity expansion used here. The equations that determine the next
order terms are derived and solved numerically for fairly realistic superfluid
neutron-star models. The damping of these modes by superfluid ``mutual
friction'' (which vanishes at the lowest order in this expansion) is found to
have a characteristic time-scale of about 10^4 s for the m=2 r-mode in a
``typical'' superfluid neutron-star model. This time-scale is far too long to
allow mutual friction to suppress the recently discovered gravitational
radiation driven instability in the r-modes. However, the strength of the
mutual friction damping depends very sensitively on the details of the
neutron-star core superfluid. A small fraction of the presently acceptable
range of superfluid models have characteristic mutual friction damping times
that are short enough (i.e. shorter than about 5 s) to suppress the
gravitational radiation driven instability completely.Comment: 15 pages, 8 figure
Relativistic Stellar Pulsations With Near-Zone Boundary Conditions
A new method is presented here for evaluating approximately the pulsation
modes of relativistic stellar models. This approximation relies on the fact
that gravitational radiation influences these modes only on timescales that are
much longer than the basic hydrodynamic timescale of the system. This makes it
possible to impose the boundary conditions on the gravitational potentials at
the surface of the star rather than in the asymptotic wave zone of the
gravitational field. This approximation is tested here by predicting the
frequencies of the outgoing non-radial hydrodynamic modes of non-rotating
stars. The real parts of the frequencies are determined with an accuracy that
is better than our knowledge of the exact frequencies (about 0.01%) except in
the most relativistic models where it decreases to about 0.1%. The imaginary
parts of the frequencies are determined with an accuracy of approximately M/R,
where M is the mass and R is the radius of the star in question.Comment: 10 pages (REVTeX 3.1), 5 figs., 1 table, fixed minor typos, published
in Phys. Rev. D 56, 2118 (1997
Second-order rotational effects on the r-modes of neutron stars
Techniques are developed here for evaluating the r-modes of rotating neutron
stars through second order in the angular velocity of the star. Second-order
corrections to the frequencies and eigenfunctions for these modes are evaluated
for neutron star models. The second-order eigenfunctions for these modes are
determined here by solving an unusual inhomogeneous hyperbolic boundary-value
problem. The numerical techniques developed to solve this unusual problem are
somewhat non-standard and may well be of interest beyond the particular
application here. The bulk-viscosity coupling to the r-modes, which appears
first at second order, is evaluated. The bulk-viscosity timescales are found
here to be longer than previous estimates for normal neutron stars, but shorter
than previous estimates for strange stars. These new timescales do not
substantially affect the current picture of the gravitational radiation driven
instability of the r-modes either for neutron stars or for strange stars.Comment: 13 pages, 5 figures, revte
A self-consistent method to estimate the rate of compact binary coalescences with a Poisson mixture model
The recently published GWTC-1 (Abbott B P et al (LIGO Scientific Collaboration and Virgo Collaboration) 2019 Phys. Rev. X 9 031040)—a journal article summarizing the search for gravitational waves (GWs) from coalescing compact binaries in data produced by the LIGO-Virgo network of ground-based detectors during their first and second observing runs—quoted estimates for the rates of binary neutron star, neutron star black hole binary, and binary black hole mergers, as well as assigned probabilities of astrophysical origin for various significant and marginal GW candidate events. In this paper, we delineate the formalism used to compute these rates and probabilities, which assumes that triggers above a low ranking statistic threshold, whether of terrestrial or astrophysical origin, occur as independent Poisson processes. In particular, we include an arbitrary number of astrophysical categories by redistributing, via mass-based template weighting, the foreground probabilities of candidate events, across source classes. We evaluate this formalism on synthetic GW data, and demonstrate that this method works well for the kind of GW signals observed during the first and second observing runs
A self-consistent method to estimate the rate of compact binary coalescences with a Poisson mixture model
The recently published GWTC-1 - a journal article summarizing the search for
gravitational waves (GWs) from coalescing compact binaries in data produced by
the LIGO-Virgo network of ground-based detectors during their first and second
observing runs - quoted estimates for the rates of binary neutron star, neutron
star black hole binary, and binary black hole mergers, as well as assigned
probabilities of astrophysical origin for various significant and marginal GW
candidate events. In this paper, we delineate the formalism used to compute
these rates and probabilities, which assumes that triggers above a low ranking
statistic threshold, whether of terrestrial or astrophysical origin, occur as
independent Poisson processes. In particular, we include an arbitrary number of
astrophysical categories by redistributing, via mass-based template weighting,
the foreground probabilities of candidate events, across source classes. We
evaluate this formalism on synthetic GW data, and demonstrate that this method
works well for the kind of GW signals observed during the first and second
observing runs.Comment: 19 pages, 5 figure
A self-consistent method to estimate the rate of compact binary coalescences with a Poisson mixture model
The recently published GWTC-1 - a journal article summarizing the search for
gravitational waves (GWs) from coalescing compact binaries in data produced by
the LIGO-Virgo network of ground-based detectors during their first and second
observing runs - quoted estimates for the rates of binary neutron star, neutron
star black hole binary, and binary black hole mergers, as well as assigned
probabilities of astrophysical origin for various significant and marginal GW
candidate events. In this paper, we delineate the formalism used to compute
these rates and probabilities, which assumes that triggers above a low ranking
statistic threshold, whether of terrestrial or astrophysical origin, occur as
independent Poisson processes. In particular, we include an arbitrary number of
astrophysical categories by redistributing, via mass-based template weighting,
the foreground probabilities of candidate events, across source classes. We
evaluate this formalism on synthetic GW data, and demonstrate that this method
works well for the kind of GW signals observed during the first and second
observing runs.Comment: 19 pages, 5 figure
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