932 research outputs found
CCOs and the hidden magnetic field scenario
CCOs are X-ray sources lying close the center of supernova remnants, with
inferred values of the surface magnetic fields significantly lower (less than
about 1e11 G) than those of standard pulsars. In this paper, we revise the
hidden magnetic field scenario, presenting the first 2D simulations of the
submergence and reemergence of the magnetic field in the crust of a neutron
star. A post-supernova accretion stage of about 1e-4-1e-3 solar masses over a
vast region of the surface is required to bury the magnetic field into the
inner crust. When accretion stops, the field reemerges on a typical timescale
of 1-100 kyr, depending on the submergence conditions. After this stage, the
surface magnetic field is restored close to its birth values. A possible
observable consequence of the hidden magnetic field is the anisotropy of the
surface temperature distribution, in agreement with observations of several of
these sources. We conclude that the hidden magnetic field model is viable as
alternative to the anti-magnetar scenario, and it could provide the missing
link between CCOs and the other classes of isolated neutron stars.Comment: 7 pages, 7 figures, MNRA
Spectral boundary conditions and solitonic solutions in a classical Sellmeier dielectric
Electromagnetic field interactions in a dielectric medium represent a
longstanding field of investigation, both at the classical level and at the
quantum one. We propose a 1+1 dimensional toy-model which consists of an
half-line filling dielectric medium, with the aim to set up a simplified
situation where technicalities related to gauge invariance and, as a
consequence, physics of constrained systems are avoided, and still interesting
features appear. In particular, we simulate the electromagnetic field and the
polarization field by means of two coupled scalar fields ,
respectively, in a Hopfield-like model. We find that, in order to obtain a
physically meaningful behaviour for the model, one has to introduce spectral
boundary conditions depending on the particle spectrum one is dealing with.
This is the first interesting achievement of our analysis. The second relevant
achievement is that, by introducing a nonlinear contribution in the
polarization field , with the aim of mimicking a third order nonlinearity
in a nonlinear dielectric, we obtain solitonic solutions in the Hopfield model
framework, whose classical behaviour is analyzed too.Comment: 12 pages, 1 figur
Risk factors for central venous catheter-related infections in surgical and intensive care units. The Central Venous Catheter-Related Infections Study Group.
To identify avoidable risk factors for central venous catheter (CVC) infections in patients undergoing short-term catheterization
Population synthesis of isolated Neutron Stars with magneto--rotational evolution
We revisit the population synthesis of isolated radio-pulsars incorporating
recent advances on the evolution of the magnetic field and the angle between
the magnetic and rotational axes from new simulations of the magneto-thermal
evolution and magnetosphere models, respectively. An interesting novelty in our
approach is that we do not assume the existence of a death line. We discuss
regions in parameter space that are more consistent with the observational
data. In particular, we find that any broad distribution of birth spin periods
with s can fit the data, and that if the alignment angle is
allowed to vary consistently with the torque model, realistic magnetospheric
models are favoured compared to models with classical magneto-dipolar radiation
losses. Assuming that the initial magnetic field is given by a lognormal
distribution, our optimal model has mean strength with width .
However, there are strong correlations between parameters. This degeneracy in
the parameter space can be broken by an independent estimate of the pulsar
birth rate or by future studies correlating this information with the
population in other observational bands (X-rays and -rays).Comment: 10 pages, 9 figures, submitted and accepted to MNRAS, comments
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Triggering magnetar outbursts in 3D force-free simulations
In this letter, we present the first 3D force-free general relativity
simulations of the magnetosphere dynamics related to the magnetar
outburst/flare phenomenology. Starting from an initial dipole configuration, we
adiabatically increase the helicity by twisting the footprints of a spot on the
stellar surface and follow the succession of quasi-equilibrium states until a
critical twist is reached. Twisting beyond that point triggers instabilities
that results in the rapid expansion of magnetic field lines, followed by
reconnection, as observed in previous axi-symmetric simulations. If the
injection of magnetic helicity goes on, the process is recurrent, periodically
releasing a similar amount of energy, of the order of a few % of the total
magnetic energy. From our current distribution, we estimate the local
temperature assuming that dissipation occurs mainly in the highly resistive
outermost layer of the neutron star. We find that the temperature smoothly
increases with injected twist, being larger for spots located in the tropical
regions than in polar regions, and rather independent of their sizes. After the
injection of helicity ceases, the magnetosphere relaxes to a new stable state,
in which the persistent currents maintain the footprints area slightly hotter
than before the onset of the instability.Comment: 6 pages, 5 figure
Spectral features in isolated neutron stars induced by inhomogeneous surface temperatures
The thermal X-ray spectra of several isolated neutron stars display
deviations from a pure blackbody. The accurate physical interpretation of these
spectral features bears profound implications for our understanding of the
atmospheric composition, magnetic field strength and topology, and equation of
state of dense matter. With specific details varying from source to source,
common explanations for the features have ranged from atomic transitions in the
magnetized atmospheres or condensed surface, to cyclotron lines generated in a
hot ionized layer near the surface. Here we quantitatively evaluate the X-ray
spectral distortions induced by inhomogeneous temperature distributions of the
neutron star surface. To this aim, we explore several surface temperature
distributions, we simulate their corresponding general relativistic X-ray
spectra (assuming an isotropic, blackbody emission), and fit the latter with a
single blackbody model. We find that, in some cases, the presence of a spurious
'spectral line' is required at a high significance level in order to obtain
statistically acceptable fits, with central energy and equivalent width similar
to the values typically observed. We also perform a fit to a specific object,
RX J0806.4-4123, finding several surface temperature distributions able to
model the observed spectrum. The explored effect is unlikely to work in all
sources with detected lines, but in some cases it can indeed be responsible for
the appearance of such lines. Our results enforce the idea that surface
temperature anisotropy can be an important factor that should be considered and
explored also in combination with more sophisticated emission models like
atmospheres.Comment: 11 pages, 7 figures; accepted for publication in MNRA
Population Synthesis of Isolated Neutron Stars with magneto-rotational evolution II: from radio-pulsars to magnetars
Population synthesis studies constitute a powerful method to reconstruct the
birth distribution of periods and magnetic fields of the pulsar population.
When this method is applied to populations in different wavelengths, it can
break the degeneracy in the inferred properties of initial distributions that
arises from single-band studies. In this context, we extend previous works to
include -ray thermal emitting pulsars within the same evolutionary model as
radio-pulsars. We find that the cumulative distribution of the number of X-ray
pulsars can be well reproduced by several models that, simultaneously,
reproduce the characteristics of the radio-pulsar distribution. However, even
considering the most favourable magneto-thermal evolution models with fast
field decay, log-normal distributions of the initial magnetic field
over-predict the number of visible sources with periods longer than 12 s. We
then show that the problem can be solved with different distributions of
magnetic field, such as a truncated log-normal distribution, or a binormal
distribution with two distinct populations. We use the observational lack of
isolated NSs with spin periods P>12 s to establish an upper limit to the
fraction of magnetars born with B > 10^{15} G (less than 1\%). As future
detections keep increasing the magnetar and high-B pulsar statistics, our
approach can be used to establish a severe constraint on the maximum magnetic
field at birth of NSs.Comment: 12 pages, 11 figures, 5 table
Pulsar timing irregularities and the imprint of magnetic field evolution
(Abridged) The rotational evolution of isolated neutron stars is dominated by
the magnetic field anchored to the solid crust of the star. Assuming that the
core field evolves on much longer timescales, the crustal field evolves mainly
though Ohmic dissipation and the Hall drift, and it may be subject to
relatively rapid changes with remarkable effects on the observed timing
properties. We investigate whether changes of the magnetic field structure and
strength during the star evolution may have observable consequences in the
braking index, which is the most sensitive quantity to reflect small variations
of the timing properties that are caused by magnetic field rearrangements. By
performing axisymmetric, long-term simulations of the magneto-thermal evolution
of neutron stars with state-of-the-art microphysical inputs, we find that the
effect of the magnetic field evolution on the braking index can be divided into
three qualitatively different stages depending on the age and the internal
temperature: a first stage that may be different for standard pulsars (with
n~3) or low field neutron stars that accreted fallback matter during the
supernova explosion (systematically n<3); in a second stage, the evolution is
governed by almost pure Ohmic field decay, and a braking index n>3 is expected;
in the third stage, at late times, when the interior temperature has dropped to
very low values, Hall oscillatory modes in the neutron star crust result in
braking indices of high absolute value and both positive and negative signs.
Models with strong (1e14 G) multipolar or toroidal components, even with a weak
(~1e12 G) dipolar field are consistent with the observed trend of the timing
properties.Comment: 7 pages, 5 figures, accepted for publication in Astronomy &
Astrophysics (submitted July 24, 2012
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