We present new simulations of the evolution of axially symmetric magnetic fields in neutron star crusts under the influence of the Hall effect and subdominant Ohmic dissipation. In the Hall effect, ...differential rotation of the electron fluid generates toroidal field by winding of the poloidal field. For this reason, we focus on the influence of the initial choice of the electron angular velocity profile on the subsequent and long-term magnetic evolution. Whereas previous simulations have generally chosen angular velocities increasing outwards, corresponding to the lowest order Ohmic mode in the crust; a more realistic choice is an angular velocity decreasing outwards, corresponding to the magnetohydrodynamic equilibrium field that is likely present at the time of crust formation. We find that the evolution passes through three basic phases. The early evolution is a response to the initial conditions. During the second phase, the field consists of poloidal and toroidal components which eventually relax to an isorotation state in which the angular velocity of the electrons becomes constant along poloidal magnetic field lines, causing Hall evolution to saturate. In the third phase, the field dissipates slowly while maintaining isorotation. We discuss the implications for the long-term field structure and observable properties of isolated neutron stars.
ABSTRACT
Under normal conditions in a neutron-star (NS) crust, ions are locked in place in the crustal lattice and only electrons are mobile, and magnetic-field evolution is thus directly related to ...the electron velocity. The evolution, however, builds magnetic stresses that can become sufficiently large for the crust to exceed its elastic limit, and to flow plastically. We consider the nature of this plastic flow and the back-reaction on the crustal magnetic-field evolution. We formulate a plane-parallel model for the local failure, showing that surface motions are inevitable once the crust yields, in the absence of extra dissipative mechanisms. We perform numerical evolutions of the crustal magnetic field under the joint effect of Hall drift and Ohmic decay, tracking the build-up of magnetic stresses, and diagnosing crustal failure with the von Mises criterion. Beyond this point we solve for the coupled evolution of the plastic velocity and magnetic field. Our results suggest that to have a coexistence of a magnetar corona with small-scale magnetic features, the viscosity of the plastic flow must be roughly 1036–1037 g cm−1s−1. We find significant motion at the surface at a rate of 10–100 cm yr−1, and that the localized magnetic field is weaker than in evolutions without plastic flow. We discuss astrophysical implications, and how our local simulations could be used to build a global model of field evolution in the NS crust.
Braking index measurements of young radio pulsars are all smaller than the value expected for spin-down by magnetic dipole braking. We investigate magnetic field evolution in the neutron star crust ...due to Hall drift as an explanation for observed braking indices. Using numerical simulations and a semi-analytic model, we show that an ≈1014 G quadrupolar toroidal field in the neutron star crust at birth leads to growth of the dipole moment at a rate large enough to agree with measured braking indices. A key factor is the density at which the crust yields to magnetic stresses that build up during the evolution, which sets a characteristic minimum Hall time-scale. The observed braking indices of pulsars with inferred dipole fields of ≲ 1013 G can be explained in this picture, although with a significant octupole component needed in some cases. For the stronger field pulsars, those with B
d ≳ 1013 G, we find that the magnetic stresses in the crust exceed the maximum shear stress before the pulsar reaches its current age, likely quenching the Hall effect. This may have implications for the magnetar activity seen in the high magnetic field radio pulsar PSR J1846−0258. Observations of braking indices may therefore be a new piece of evidence that neutron stars contain subsurface toroidal fields that are significantly stronger than the dipole field, and may indicate that the Hall effect is important in a wider range of neutron stars than previously thought.
ABSTRACT
While the dipole magnetic field axis of neutron stars is usually postulated to cross the star’s centre, it may be displaced from this location, as it has been recently indicated in the ...millisecond pulsar J0030+0451. Under these conditions, the electromagnetic rocket effect may be activated, where the magnetic field exerts a net force, accelerating the star. This post-natal kick mechanism relies on asymmetric electromagnetic radiation from an off-centre dipole and may be relevant to the high spatial velocities of pulsars $\sim 10^{3}$ km s−1. Here, we explore its impact in young pulsars associated with supernova remnants, and we compare the observational data on characteristic quantities, such as the braking index and proper motion, with results obtained from the rocket effect. Using a Markov Chain Monte Carlo analysis, we explore the required conditions, for the initial spin periods and the distance between the magnetic axis and the star’s centre, so that the velocity kick due to the rocket effect approaches the present velocity. We find that the electromagnetic rocket effect can account for typical pulsar transverse velocities assuming an initial spin period of 3.8 $\rm {ms}$ and a dipole field whose distance from the centre of the star is approximately 7 $\rm {km}$ . We also explore the influence of the rocket effect on the braking index of a neutron star, and we find that for the sample studied this impact is minimal. Finally, we apply the rocket effect model on the pulsars J0030+0451 and J0538+2817, which are likely candidates for this mechanism.
We present a timing and glitch analysis of the young X-ray pulsar PSR J0537−6910, located within the Large Magellanic Cloud, using 13 yr of data from the now-decommissioned Rossi X-ray Timing ...Explorer. Rotating with a spin period of 16 ms, PSR J0537−6910 is the fastest-spinning and most energetic young pulsar known. It also displays the highest glitch activity of any known pulsar. We have found 42 glitches over the data span, corresponding to a glitch rate of 3.2 yr−1, with an overall glitch activity rate of . The high glitch frequency has allowed us to study the glitch behavior in ways that are inaccessible in other pulsars. We observe a strong linear correlation between spin frequency glitch magnitude and wait time to the following glitch. We also find that the post-glitch spin-down recovery is well described by a single two-component model fit to all glitches for which we have adequate input data. This consists of an exponential amplitude , decay timescale s, and linear slope . The latter slope corresponds to a second frequency derivative , from which we find an implied braking index . We also present a maximum likelihood technique for searching for periods in event-time data, which we used to both confirm previously published values and determine rotation frequencies in later observations. We discuss the implied constraints on glitch models from the observed behavior of this system, which we argue cannot be fully explained in the context of existing theories.
Twisted magnetar magnetospheres Ntotsikas, D; Gourgouliatos, K N; Contopoulos, I ...
Monthly Notices of the Royal Astronomical Society,
01/2024, Volume:
527, Issue:
3
Journal Article
Peer reviewed
Open access
ABSTRACT
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.
We present solutions for Hall equilibria applicable to neutron star crusts. Such magnetic configurations satisfy a Grad-Shafranov-type equation, which is solved analytically and numerically. The ...solutions presented cover a variety of configurations, from purely poloidal fields connected to an external dipole to poloidal-toroidal fields connected to an external vacuum field, or fully confined within the star. We find that a dipole external field should be supported by a uniformly rotating electron fluid. The energy of the toroidal magnetic field is generally found to be a few per cent of the total magnetic field energy for the fields with an external component. We discuss the evolution due to Ohmic dissipation which leads to slowing down of the electron fluid. We also find that the transition from an MHD equilibrium to a state governed by Hall effect generates spontaneously an additional toroidal field in regions where the electron fraction changes.
Current closure through the neutron star crust Karageorgopoulos, V; Gourgouliatos, K N; Contopoulos, I
Monthly notices of the Royal Astronomical Society,
08/2019, Volume:
487, Issue:
3
Journal Article
Peer reviewed
ABSTRACT
Force-free pulsar magnetospheres develop a large-scale poloidal electric current circuit that flows along open magnetic field lines from the neutron star to the termination shock. The ...electric current closes through the interior of the neutron star where it provides the torque that spins-down the star. In the present work, we study the internal electric current in an axisymmetric rotator. We evaluate the path of the electric current by requiring the minimization of internal Ohmic losses. We find that, in millisecond pulsars, the current reaches the base of the crust, while in pulsars with periods of a few seconds, the bulk of the electric current does not penetrate deeper than about 100 m. The region of maximum spin-down torque in millisecond pulsars is the base of the crust, while in slowly spinning ones it is the outer crust. We evaluate the corresponding Maxwell stresses and find that, in typical rotation-powered radio pulsars, they are well below the critical stress that can be sustained by the crust. For magnetar-level fields, the Maxwell stresses near the surface are comparable to the critical stress and may lead to the decoupling of the crust from the rest of the stellar rotation.
An anti-glitch in a magnetar Archibald, R F; Kaspi, V M; Ng, C-Y ...
Nature (London),
05/2013, Volume:
497, Issue:
7451
Journal Article
Peer reviewed
Magnetars are neutron stars with X-ray and soft γ-ray outbursts thought to be powered by intense internal magnetic fields. Like conventional neutron stars in the form of radio pulsars, magnetars ...exhibit 'glitches' during which angular momentum is believed to be transferred between the solid outer crust and the superfluid component of the inner crust. The several hundred observed glitches in radio pulsars and magnetars have involved a sudden spin-up (increase in the angular velocity) of the star, presumably because the interior superfluid was rotating faster than the crust. Here we report X-ray timing observations of the magnetar 1E 2259+586 (ref. 8), which exhibited a clear 'anti-glitch'--a sudden spin-down. We show that this event, like some previous magnetar spin-up glitches, was accompanied by multiple X-ray radiative changes and a significant spin-down rate change. Such behaviour is not predicted by models of neutron star spin-down and, if of internal origin, is suggestive of differential rotation in the magnetar, supporting the need for a rethinking of glitch theory for all neutron stars.
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DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We present an analytical class of equilibrium solutions for the structure of relativistic sheared and rotating magnetized jets that contain no boundary current sheets. We demonstrate the overall ...dynamical stability of these solutions and, most importantly, a better numerical resistive stability than the commonly employed force-free structures which inevitably require the presence of dissipative surface currents. The jet is volumetrically confined by the external pressure, with no pressure gradient on the surface. We calculate the expected observed properties of such jets. Given the simplicity of these solutions, we suggest them as useful initial conditions for relativistic jet simulations.
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