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 $\beta-$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, $T \lesssim 1-2\times10^8$ 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 $T \lesssim 10^9$ 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 $10^{45}\,$erg for fields of $10^{14}\,$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 $\sim 0.1\,$keV to $\sim 0.3 - 0.6\,$keV, in good agreement with observations. Similarly, the power input from the interior into the magnetosphere could be as high as $10^{35} - 10^{36}\,$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 $\Lambda$ and ellipticity $\epsilon$ 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 $4\times 10^8$ T) to emit detectable gravitational radiation, the neutron star must have a strong toroidal field component, with maximum internal toroidal field strength $B_{\mathrm{tm}}=7\times 10^{12}$ T; for gravitational waves to be detected from the Cas A CCO at 300 Hz, $B_{\mathrm{tm}}\sim 10^{13}$ T, whereas detection at 100 Hz would require $B_{\mathrm{tm}}\sim 10^{14}$ 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 $\Lambda\lesssim 0.2$) 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