284 research outputs found
The occurrence of Guillain-Barré syndrome within families (multiple letters)
We thank Drs. Korn-Lubetzki and Steiner for their interest in our article. They concluded that the 17p12 deletion responsible for HNPP was also present in a family in which three members had an IDP. Two members fulfilled the criteria for CIDP and the other for AIDP.This finding may indicate that CIDP and this deletion may also be present in our recently reported Dutch families in which two or more members had GBS. [...
Magneto-elastic torsional oscillations of magnetars
We extend a general-relativistic ideal magneto-hydrodynamical code to include
the effects of elasticity. Using this numerical tool we analyse the
magneto-elastic oscillations of highly magnetised neutron stars (magnetars). In
simulations without magnetic field we are able to recover the purely crustal
shear oscillations within an accuracy of about a few per cent. For dipole
magnetic fields between 5 x 10^13 and 10^15 G the Alfv\'en oscillations become
modified substantially by the presence of the crust. Those quasi-periodic
oscillations (QPOs) split into three families: Lower QPOs near the equator,
Edge QPOs related to the last open field line and Upper QPOs at larger distance
from the equator. Edge QPOs are called so because they are related to an edge
in the corresponding Alfv\'en continuum. The Upper QPOs are of the same kind,
while the Lower QPOs are turning-point QPOs, related to a turning point in the
continuous spectrum.Comment: 6 pages, 1 figure, 1 table, Proceedings of NEB14, to appear in J.
Phys.: Conf. Se
Magnetar Oscillations I: strongly coupled dynamics of the crust and the core
Quasi-Periodic Oscillations (QPOs) observed during Soft Gamma Repeaters giant
flares are commonly interpreted as the torsional oscillations of magnetars. The
oscillatory motion is influenced by the strong interaction between the shear
modes of the crust and Alfven-like modes in the core. We study the dynamics
which arises through this interaction, and present several new results: (1) We
show that global {\it edge modes} frequently reside near the edges of the core
Alfven continuum. (2) We compute the magnetar's oscillatory motion for
realistic axisymmetric magnetic field configurations and core density profiles,
but with a simplified model of the elastic crust. We show that one may
generically get multiple gaps in the Alfven continuum. One obtains discrete
global {\it gap modes} if the crustal frequencies belong to the gaps. (3) We
show that field tangling in the core enhances the role of the core discrete
Alfven modes and reduces the role of the core Alfven continuum in the overall
oscillatory dynamics of the magnetar. (4) We demonstrate that the system
displays transient and/or drifting QPOs when parts of the spectrum of the core
Alfven modes contain discrete modes which are densely and regularly spaced in
frequency. (5) We show that if the neutrons are coupled into the core Alfven
motion, then the post-flare crustal motion is strongly damped and has a very
weak amplitude. Thus magnetar QPOs give evidence that the proton and neutron
components in the core are dynamically decoupled and that at least one of them
is a quantum fluid. (6) We show that it is difficult to identify the
high-frequency 625 Hz QPO as being due to the physical oscillatory mode of the
magnetar, if the latter's fluid core consists of the standard
proton-neutron-electron mixture and is magnetised to the same extent as the
crust. (Abstract abridged)Comment: 22 pages, 22 figures, submitted to MNRA
Magnetar Oscillations II: spectral method
The seismological dynamics of magnetars is largely determined by a strong
hydro-magnetic coupling between the solid crust and the fluid core. In this
paper we set up a "spectral" computational framework in which the magnetar's
motion is decomposed into a series of basis functions which are associated with
the crust and core vibrational eigenmodes. A general-relativistic formalism is
presented for evaluation of the core Alfven modes in the magnetic-flux
coordinates, as well for eigenmode computation of a strongly magnetized crust
of finite thickness. By considering coupling of the crustal modes to the
continuum of Alfven modes in the core, we construct a fully relativistic
dynamical model of the magnetar which allows: i) Fast and long simulations
without numerical dissipation. ii) Very fine sampling of the stellar structure.
We find that the presence of strong magnetic field in the crust results in
localizing of some high-frequency crustal elasto-magnetic modes with the radial
number n>1 to the regions of the crust where the field is nearly horizontal.
While the hydro-magnetic coupling of these localized modes to the Alfven
continuum in the core is reduced, their energy is drained on a time-scale much
less than 1 second. Therefore the puzzle of the observed QPOs with frequencies
larger than 600 Hz still stands.Comment: 15 pages, 11 figures, submitted to MNRA
Measuring the neutron star equation of state using X-ray timing
One of the primary science goals of the next generation of hard X-ray timing
instruments is to determine the equation of state of the matter at supranuclear
densities inside neutron stars, by measuring the radius of neutron stars with
different masses to accuracies of a few percent. Three main techniques can be
used to achieve this goal. The first involves waveform modelling. The flux we
observe from a hotspot on the neutron star surface offset from the rotational
pole will be modulated by the star's rotation, giving rise to a pulsation.
Information about mass and radius is encoded into the pulse profile via
relativistic effects, and tight constraints on mass and radius can be obtained.
The second technique involves characterising the spin distribution of accreting
neutron stars. The most rapidly rotating stars provide a very clean constraint,
since the mass-shedding limit is a function of mass and radius. However the
overall spin distribution also provides a guide to the torque mechanisms in
operation and the moment of inertia, both of which can depend sensitively on
dense matter physics. The third technique is to search for quasi-periodic
oscillations in X-ray flux associated with global seismic vibrations of
magnetars (the most highly magnetized neutron stars), triggered by magnetic
explosions. The vibrational frequencies depend on stellar parameters including
the dense matter equation of state. We illustrate how these complementary X-ray
timing techniques can be used to constrain the dense matter equation of state,
and discuss the results that might be expected from a 10m instrument. We
also discuss how the results from such a facility would compare to other
astronomical investigations of neutron star properties. [Modified for arXiv]Comment: To appear in Reviews of Modern Physics as a Colloquium, 23 pages, 9
figure
Upper limits on the observational effects of nuclear pasta in neutron stars
The effects of the existence of exotic nuclear shapes at the bottom of the
neutron star inner crust - nuclear `pasta' - on observational phenomena are
estimated by comparing the limiting cases that those phases have a vanishing
shear modulus and that they have the shear modulus of a crystalline solid . We
estimate the effect on torsional crustal vibrations and on the maximum
quadrupole ellipticity sustainable by the crust. The crust composition and
transition densities are calculated consistently with the global properties,
using a liquid drop model with a bulk nuclear equation of state (EoS) which
allows a systematic variation of the nuclear symmetry energy. The symmetry
energy J and its density dependence L at nuclear saturation density are the
dominant nuclear inputs which determine the thickness of the crust, the range
of densities at which pasta might appear, as well as global properties such as
the radius and moment of inertia. We show the importance of calculating the
global neutron star properties on the same footing as the crust EoS, and
demonstrate that in the range of experimentally acceptable values of L, the
pasta phase can alter the crust frequencies by up to a factor of three,
exceeding the effects of superfluidity on the crust modes, and decrease the
maximum quadrupole ellipticity sustainable by the crust by up to an order of
magnitude. The signature of the pasta phases and the density dependence of the
symmetry energy on the potential observables highlights the possibility of
constraining the EoS of dense, neutron-rich matter and the properties of the
pasta phases using astrophysical observations.Comment: 8 pages, 7 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Models of hydrostatic magnetar atmospheres at high luminosities
We investigate the possibility of Photospheric Radius Expansion (PRE) during
magnetar bursts. Identification of PRE would enable a determination of the
magnetic Eddington limit (which depends on field strength and neutron star mass
and radius), and shed light on the burst mechanism. To do this we model
hydrostatic atmospheres in a strong radial magnetic field, determining both
their maximum extent and photospheric temperatures. We find that
spatially-extended atmospheres cannot exist in such a field configuration:
typical maximum extent for magnetar-strength fields is ~10 m (as compared to
200 km in the non-magnetic case). Achieving balance of gravitational and
radiative forces over a large range of radii, which is critical to the
existence of extended atmospheres, is rendered impossible in strong fields due
to the dependence of opacities on temperature and field strength. We conclude
that high luminosity bursts in magnetars do not lead to expansion and cooling
of the photosphere, as in the non-magnetic case. We also find the maximum
luminosity that can propagate through a hydrostatic magnetar atmosphere to be
lower than previous estimates. The proximity and small extent of the
photospheres associated with the two different polarization modes also calls
into question the interpretation of two blackbody fits to magnetar burst
spectra as being due to extended photospheres.Comment: Accepted for publication in MNRAS. 14 pages, 6 figures, 2 table
Two-fluid models of superfluid neutron star cores
Both relativistic and non-relativistic two-fluid models of neutron star cores
are constructed, using the constrained variational formalism developed by
Brandon Carter and co-workers. We consider a mixture of superfluid neutrons and
superconducting protons at zero temperature, taking into account mutual
entrainment effects. Leptons, which affect the interior composition of the
neutron star and contribute to the pressure, are also included. We provide the
analytic expression of the Lagrangian density of the system, the so-called
master function, from which the dynamical equations can be obtained. All the
microscopic parameters of the models are calculated consistently using the
non-relativistic nuclear energy density functional theory. For comparison, we
have also considered relativistic mean field models. The correspondence between
relativistic and non-relativistic hydrodynamical models is discussed in the
framework of the recently developed 4D covariant formalism of Newtonian
multi-fluid hydrodynamics. We have shown that entrainment effects can be
interpreted in terms of dynamical effective masses that are larger in the
relativistic case than in the Newtonian case. With the nuclear models
considered in this work, we have found that the neutron relativistic effective
mass is even greater than the bare neutron mass in the liquid core of neutron
stars.Comment: 24 pages, 15 figures, accepted for publication in MNRA
Constraints on Pasta Structure of Neutron Stars from Oscillations in Giant Flares
We show that the shear modes in the neutron star crust are quite sensitive to
the existence of nonuniform nuclear structures, so-called "pasta". Due to the
existence of pasta phase, the frequencies of shear modes are reduced, where the
dependence of fundamental frequency is different from that of overtones. Since
the torsional shear frequencies depend strongly on the structure of pasta
phase, through the observations of stellar oscillations, one can probe the
pasta structure in the crust, although that is quite difficult via the other
observations. Additionally, considering the effect of pasta phase, we show the
possibility to explain the all observed frequencies in the SGR 1806-20 with
using only crust torsional shear modes
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