567 research outputs found
The relevance of ambipolar diffusion for neutron star evolution
We study ambipolar diffusion in strongly magnetised neutron stars, with
special focus on the effects of neutrino reaction rates and the impact of a
superfluid/superconducting transition in the neutron star core. For
axisymmetric magnetic field configurations, we determine the deviation from
equilibrium induced by the magnetic force and calculate the velocity of
the slow, quasi-stationary, ambipolar drift. We study the temperature
dependence of the velocity pattern and clearly identify the transition to a
predominantly solenoidal flow. For stars without superconducting/superfluid
constituents and with a mixed poloidal-toroidal magnetic field of typical
magnetar strength, we find that ambipolar diffusion proceeds fast enough to
have a significant impact on the magnetic field evolution only at low core
temperatures, K. The ambipolar diffusion timescale
becomes appreciably shorter when fast neutrino reactions are present, because
the possibility to balance part of the magnetic force with pressure gradients
is reduced. We also find short ambipolar diffusion timescales in the case of
superconducting cores for K, due to the reduced interaction
between protons and neutrons. In the most favourable scenario, with fast
neutrino reactions and superconducting cores, ambipolar diffusion results in
advection velocities of several km/kyr. This velocity can substantially
reorganize magnetic fields in magnetar cores, in a way that can only be
confirmed by dynamical simulations.Comment: 14 pages, 11 figures, version accepted for publication in MNRA
The force-free twisted magnetosphere of a neutron star
We present a detailed analysis of the properties of twisted, force-free magnetospheres of non-rotating neutron stars, which are of interest in the modelling of magnetar properties and evolution. In our models the magnetic field smoothly matches to a current-free (vacuum) solution at some large external radius, and they are specifically built to avoid pathological surface currents at any of the interfaces. By exploring a large range of parameters, we find a few remarkable general trends. We find that the total dipolar moment can be increased by up to 40 per cent with respect to a vacuum model with the same surface magnetic field, due to the contribution of magnetospheric currents to the global magnetic field. Thus, estimates of the surface magnetic field based on the large-scale dipolar braking torque are slightly overestimating the surface value by the same amount. Consistently, there is a moderate increase in the total energy of the model with respect to the vacuum solution of up to 25 per cent, which would be the available energy budget in the event of a fast, global magnetospheric reorganization commonly associated with magnetar flares. We have also found the interesting result of the existence of a critical twist (φmax ≲ 1.5 rad), beyond which we cannot find any more numerical solutions. Combining the models considered in this paper with the evolution of the interior of neutron stars will allow us to study the influence of the magnetosphere on the long-term magnetic, thermal, and rotational evolution.This work is supported in part by the Spanish MINECO grants AYA2013-40979-P, AYA2013-42184-P, and AYA2015-66899-C2-2-P, the grant of Generalitat Valenciana PROMETEOII-2014-069, the European Union ERC Starting Grant 259276-CAMAP, and by the New Compstar COST action MP1304
Long-term evolution of the force-free twisted magnetosphere of a magnetar
We study the long-term quasi-steady evolution of the force-free magnetosphere of a magnetar coupled to its internal magnetic field. We find that magnetospheric currents can be maintained on long time-scales of the order of thousands of years. Meanwhile, the energy, helicity and twist stored in the magnetosphere all gradually increase over the course of this evolution, until a critical point is reached, beyond which a force-free magnetosphere cannot be constructed. At this point, some large-scale magnetospheric rearrangement, possibly resulting in an outburst or a flare, must occur, releasing a large fraction of the stored energy, helicity and twist. After that, the quasi-steady evolution should continue in a similar manner from the new initial conditions. The time-scale for reaching this critical point depends on the overall magnetic field strength and on the relative fraction of the toroidal field. The energy stored in the force-free magnetosphere is found to be up to ∼30 per cent larger than the corresponding vacuum energy. This implies that for a 1014 G field at the pole, the energy budget available for fast magnetospheric events is of the order of a few 1044 erg. The spin-down rate is estimated to increase by up to ∼60 per cent, since the dipole content in the magnetosphere is enhanced by the currents present there. A rough estimate of the braking index n reveals that it is systematically n < 3 for the most part of the evolution, consistent with actual measurements for pulsars and early estimates for several magnetars.This work is supported in part by the Spanish MINECO grants AYA2015-66899-C2-1-P, AYA2015-66899-C2-2-P, the grant of Generalitat Valenciana PROMETEOII-2014-069 and by the New Compstar COST action MP1304
Assessment of kinematic rock slope failures in Mudurnu Valley, Turkey
Slope instabilities are one of the most frequent natural hazards capable of causing severe failures both at regional and large scales. Mudurnu, which is settled on a steep valley, is affected by regional rock slope instabilities. These instabilities constitute a hazard and create an important risk to the community since they threaten human lives, settlement areas, and historically-important structures. In order to minimize the hazard and risk associated with slope instabilities, rock masses along the valley were characterized and the potential failure mechanisms were defined. The west side of the valley, which is the focus of the research, is characterized by Cretaceous pelagic discontinuous limestone, and is prone to complex failures. The aim of the study is to characterize the rock mass along the valley, divide the area into geomechanically-uniform sectors, define possible modes of failure (kinematics) and ultimately quantify the potential failure (kinetics) and the associated risk. For the study, in addition to the field work and scan-line survey measurements, an Unmanned Aerial Vehicle (UAV) was utilized to collect high-resolution images from problematic locations that were not accessible. Then, a point cloud of the area was generated. The images were interpreted and the resulting structural representation of the rock mass was compared with information gathered from the scan-line survey in the field. Afterwards, it was used to identify the possible modes of failure along the valley. Since seismic activity in the area is significant due to the proximity of the North Anatolian Fault Zone (NAFZ), which is the most active fault system in Turkey, dynamic loading was also considered for the stability analyses
Crust-magnetosphere coupling during magnetar evolution and implications for the surface temperature
We study the coupling of the force-free magnetosphere to the long-term
internal evolution of a magnetar. We allow the relation between the poloidal
and toroidal stream functions - that characterizes the magnetosphere - to
evolve freely without constraining its particular form. We find that, on
time-scales of the order of kyr, the energy stored in the magnetosphere
gradually increases, as the toroidal region grows and the field lines expand
outwards. This continues until a critical point is reached beyond which
force-free solutions for the magnetosphere can no longer be constructed, likely
leading to some large-scale magnetospheric reorganization. The energy budget
available for such events can be as high as several erg for fields
of G. Subsequently, starting from the new initial conditions, the
evolution proceeds in a similar manner. The time-scale to reach the critical
point scales inversely with the magnetic field amplitude. Allowing currents to
pass through the last few meters below the surface, where the magnetic
diffusivity is orders of magnitude larger than in the crust, should give rise
to a considerable amount of energy deposition through Joule heating. We
estimate that the effective surface temperature could increase locally from
keV to keV, in good agreement with observations.
Similarly, the power input from the interior into the magnetosphere could be as
high as erg/s, which is consistent with peak luminosities
observed during magnetar outbursts. Therefore, a detailed treatment of currents
flowing through the envelope may be needed to explain the thermal properties of
magnetars.Comment: 9 pages, 9 figures; updated version accepted for publication by MNRA
Gravitational wave emission from a magnetically deformed non-barotropic neutron star
A strong candidate for a source of gravitational waves is a highly
magnetised, rapidly rotating neutron star (magnetar) deformed by internal
magnetic stresses. We calculate the mass quadrupole moment by perturbing a
zeroth-order hydrostatic equilibrium by an axisymmetric magnetic field with a
\emph{linked poloidal-toroidal structure}. In this work, we do \emph{not}
require the model star to obey a barotropic equation of state (as a realistic
neutron star is not barotropic), allowing us to explore the hydromagnetic
equilibria with fewer constraints. We derive the relation between the ratio of
poloidal-to-total field energy and ellipticity and briefly
compare our results to those obtained using the barotropic assumption. Then, we
present some examples of how our results can be applied to astrophysical
contexts. First, we show how our formulae, in conjunction with current
gravitational wave (non-)detections of the Crab pulsar and the Cassiopeia A
central compact object (Cas A CCO), can be used to constrain the strength of
the internal toroidal fields of those objects. We find that, for the Crab
pulsar (whose canonical equatorial dipole field strength, inferred from spin
down, is T) to emit detectable gravitational radiation, the
neutron star must have a strong toroidal field component, with maximum internal
toroidal field strength T; for gravitational
waves to be detected from the Cas A CCO at 300 Hz, T, whereas detection at 100 Hz would require T. Using our results, we also show how the gravitational wave signal
emitted by a magnetar immediately after its birth (assuming it is born rapidly
rotating, with ) makes such a newborn magnetar a stronger
candidate for gravitational wave detection than, for example, an SGR giant
flare.Comment: 15 pages, 8 figures, 2 table
Current clinician perspective on non-vitamin K antagonist oral anticoagulant use in challenging clinical cases.
OBJECTIVE: The evolution of non-vitamin K antagonist anticoagulants (NOACs) has changed the horizon of stroke prevention in atrial fibrillation (SPAF). All 4 NOACs have been tested against dose-adjusted warfarin in well-designed, pivotal, phase III, randomized, controlled trials (RCTs) and were approved by regulatory authorities for an SPAF indication. However, as traditional RCTs, these trials have important weaknesses, largely related to their complex structure and patient participation, which was limited by strict inclusion and extensive exclusion criteria. In the real world, however, clinicians are often faced with complex, multimorbid patients who are underrepresented in these RCTs. This article is based on a meeting report authored by 12 scientists studying atrial fibrillation (AF) in diverse ways who discussed the management of challenging AF cases that are underrepresented in pivotal NOAC trials. METHODS: An advisory board panel was convened to confer on management strategies for challenging AF cases. The article is derived from a summary of case presentations and the collaborative discussions at the meeting. CONCLUSION: This expert consensus of cardiologists aimed to define management strategies for challenging cases with patients who underrepresented in pivotal trials using case examples from their routine practice. Although strong evidence is lacking, exploratory subgroup analysis of phase III pivotal trials partially informs the management of these patients. Clinical trials with higher external validity are needed to clarify areas of uncertainty. The lack of clear evidence about complex AF cases has pushed clinicians to manage patients based on clinical experience, including rare situations of off-label prescriptions
Lattice dynamics and structural stability of ordered Fe3Ni, Fe3Pd and Fe3Pt alloys
We investigate the binding surface along the Bain path and phonon dispersion
relations for the cubic phase of the ferromagnetic binary alloys Fe3X (X = Ni,
Pd, Pt) for L12 and DO22 ordered phases from first principles by means of
density functional theory. The phonon dispersion relations exhibit a softening
of the transverse acoustic mode at the M-point in the L12-phase in accordance
with experiments for ordered Fe3Pt. This instability can be associated with a
rotational movement of the Fe-atoms around the Ni-group element in the
neighboring layers and is accompanied by an extensive reconstruction of the
Fermi surface. In addition, we find an incomplete softening in [111] direction
which is strongest for Fe3 Ni. We conclude that besides the valence electron
density also the specific Fe-content and the masses of the alloying partners
should be considered as parameters for the design of Fe-based functional
magnetic materials.Comment: Revised version, accepted for publication in Physical Review
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