9,696 research outputs found
Magnetic neutron star equilibria with stratification and type-II superconductivity
We construct two-fluid equilibrium configurations for neutron stars with
magnetic fields, using a self-consistent and nonlinear numerical approach. The
two-fluid approach - likely to be valid for large regions of all but the
youngest NSs - provides us with a straightforward way to introduce
stratification and allows for more realistic models than the ubiquitous
barotropic assumption. In all our models the neutrons are modelled as a
superfluid, whilst for the protons we consider two cases: one where they are a
normal fluid and another where they form a type-II superconductor. We consider
a variety of field configurations in the normal-proton case and purely toroidal
fields in the superconducting case. We find that stratification allows for a
stronger toroidal component in mixed-field configurations, though the poloidal
component remains the largest in all our models. We provide quantitative
results for magnetic ellipticities of NSs, both in the normal- and
superconducting-proton cases.Comment: 21 pages, 14 figures; some minor changes to match published versio
M51 ULX-7: superorbital periodicity and constraints on the neutron star magnetic field
In this work, we explore the applicability of standard theoretical models of accretion to the observed properties of M51 ULX-7. The spin-up rate and observed X-ray luminosity are evidence of a neutron star with a surface magnetic field of 2-7 x 10(13) G, rotating near equilibrium. Analysis of the X-ray light curve of the system (Swift/XRT data) reveals the presence of a similar to 39 d superorbital period. We argue that the superorbital periodicity is due to disc precession, and that material is accreted on to the neutron star at a constant rate throughout it. Moreover, by attributing this modulation to the free precession of the neutron star we estimate a surface magnetic field strength of 3-4 x 10(13) G. The agreement of these two independent estimates provide strong constraints on the surface polar magnetic field strength of the NS
The game of life on a magnetar crust: from -ray flares to FRBs
This paper presents a model to unify the diverse range of magnetar activity,
through the building and release of elastic stress from the crust. A cellular
automaton drives both local and global yielding of the crust, leading to
braiding of coronal loops and energy release. The model behaves like a real
magnetar in many ways: giant flares and small bursts both occur, as well as
periods of quiescence whose typical duration is either yr or yr. The burst energy distribution broadly follows an earthquake-like
power law over the energy range . The local nature
of coronal loops allows for the possibility of high-energy and fast radio
bursts from the same magnetar. Within this paradigm, magnetar observations can
be used to constrain the poorly-understood mechanical properties of the
neutron-star crust.Comment: 8 pages, 5 figures. Comments welcome. Animations related to figure 3
available here: sklander.wordpress.com/animations
Are there any stable magnetic fields in barotropic stars?
We construct barotropic stellar equilibria, containing magnetic fields with
both poloidal and toroidal field components. We extend earlier results by
exploring the effect of different magnetic field and current distributions. Our
results suggest that the boundary treatment plays a major role in whether the
poloidal or toroidal field component is globally dominant. Using time
evolutions we provide the first stability test for mixed poloidal-toroidal
fields in barotropic stars, finding that all these fields suffer instabilities
due to one of the field components: these are localised around the pole for
toroidal-dominated equilibria and in the closed-field line region for
poloidal-dominated equilibria. Rotation provides only partial stabilisation.
There appears to be very limited scope for the existence of stable magnetic
fields in barotropic stars. We discuss what additional physics from real stars
may allow for stable fields.Comment: 16 pages, 11 figures. Some minor revision from v1, including a new
figure; results unchanged. Now published in MNRA
Noninvasive Measurement of Dissipation in Colloidal Systems
According to Harada and Sasa [Phys. Rev. Lett. 95, 130602 (2005)], heat
production generated in a non-equilibrium steady state can be inferred from
measuring response and correlation functions. In many colloidal systems,
however, it is a nontrivial task to determine response functions, whereas
details about spatial steady state trajectories are easily accessible. Using a
simple conditional averaging procedure, we show how this fact can be exploited
to reliably evaluate average heat production. We test this method using
Brownian dynamics simulations, and apply it to experimental data of an
interacting driven colloidal system
Magnetar birth: rotation rates and gravitational-wave emission
Understanding the evolution of the angle χ between a magnetar's rotation and magnetic axes sheds light on the star's birth properties. This evolution is coupled with that of the stellar rotation ω, and depends on the competing effects of internal viscous dissipation and external torques. We study this coupled evolution for a model magnetar with a strong internal toroidal field, extending previous work by modelling-for the first time in this context-the strong protomagnetar wind acting shortly after birth. We also account for the effect of buoyancy forces on viscous dissipation at late times. Typically, we find that χ → 90° shortly after birth, then decreases towards 0° over hundreds of years. From observational indications that magnetars typically have small χ, we infer that these stars are subject to a stronger average exterior torque than radio pulsars, and that they were born spinning faster than ∼100-300 Hz. Our results allow us to make quantitative predictions for the gravitational and electromagnetic signals from a newborn rotating magnetar. We also comment briefly on the possible connection with periodic fast radio burst sources
Adult Learning in Social Innovation
A qualitative study of practitioner learning in a multi-year, cross-sector, social innovation project indicated that team-based learning associated with the collaborative program model was expansive in character and influenced organizational learning beyond the project
Matching faces with emotional expressions.
There is some evidence that faces with a happy expression are recognized better than faces with other expressions. However, little is known about whether this happy-face advantage also applies to perceptual face matching, and whether similar differences exist among other expressions. Using a sequential matching paradigm, we systematically compared the effects of seven basic facial expressions on identity recognition. Identity matching was quickest when a pair of faces had an identical happy/sad/neutral expression, poorer when they had a fearful/surprise/angry expression, and poorest when they had a disgust expression. Faces with a happy/sad/fear/surprise expression were matched faster than those with an anger/disgust expression when the second face in a pair had a neutral expression. These results demonstrate that effects of facial expression on identity recognition are not limited to happy-faces when a learned face is immediately tested. The results suggest different influences of expression in perceptual matching and long-term recognition memory
Effective Confinement as Origin of the Equivalence of Kinetic Temperature and Fluctuation-Dissipation Ratio in a Dense Shear Driven Suspension
We study response and velocity autocorrelation functions for a tagged
particle in a shear driven suspension governed by underdamped stochastic
dynamics. We follow the idea of an effective confinement in dense suspensions
and exploit a time-scale separation between particle reorganization and
vibrational motion. This allows us to approximately derive the
fluctuation-dissipation theorem in a "hybrid" form involving the kinetic
temperature as an effective temperature and an additive correction term. We
show numerically that even in a moderately dense suspension the latter is
negligible. We discuss similarities and differences with a simple toy model, a
single trapped particle in shear flow
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