8,581 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
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
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
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
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
Twisted magnetar magnetospheres
Magnetar magnetospheres are strongly twisted, and are able to power sudden
energetic events through the rapid release of stored electromagnetic energy. In
this paper, we investigate twisted relativistic force-free axisymmetric
magnetospheres of rotating neutron stars. We obtain numerical solutions of such
configurations using the method of simultaneous relaxation for the magnetic
field inside and outside the light-cylinder. We introduce a toroidal magnetic
field in the region of closed field-lines that is associated with a poloidal
electric current distribution in that region, and explore various mathematical
expressions for that distribution. We find that, by increasing the twist, a
larger fraction of magnetic field-lines crosses the light-cylinder and opens up
to infinity, thus increasing the size of the polar caps and enhancing the
spin-down rate. We also find that, for moderately to strongly twisted
magnetospheres, the region of closed field-lines ends at some distance inside
the light-cylinder. We discuss the implications of these solutions on the
variation of magnetar spin-down rates, moding and nulling of pulsars, the
relation between the angular shear and the twist and the overall shape of the
magnetosphere.Comment: Accepted by MNRA
Generating neutron-star magnetic fields: three dynamo phases
Young neutron stars (NSs) have magnetic fields in the range 1012-1015 G, believed to be generated by dynamo action at birth. We argue that such a dynamo is actually too inefficient to explain the strongest of these fields. Dynamo action in the mature star is also unlikely. Instead we propose a promising new precession-driven dynamo and examine its basic properties, as well as arguing for a revised mean-field approach to NS dynamos. The precession-driven dynamo could also play a role in field generation in main-sequence stars
Probability distribution of the maximum of a smooth temporal signal
We present an approximate calculation for the distribution of the maximum of
a smooth stationary temporal signal X(t). As an application, we compute the
persistence exponent associated to the probability that the process remains
below a non-zero level M. When X(t) is a Gaussian process, our results are
expressed explicitly in terms of the two-time correlation function,
f(t)=.Comment: Final version (1 major typo corrected; better introduction). Accepted
in Phys. Rev. Let
Spin Correlations In Actinide Materials: A Neutron Study Of USb
Measurements have been made of the short-range spin correlations in USb, a metallic compound that orders antiferromagnetically. The system has no transverse fluctuations, and the longitudinal spin correlations are anisotropic, showing stronger interactions within the ferromagnetic sheets than between them. The results of this and other experiments can be understood with simple concepts involving the bonding of 5f electrons. © 1978 The American Physical Society
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