Evolution of the magnetic field in magnetars

Braithwaite, Jonathan; Spruit, H. C.

doi:10.1051/0004-6361:20041981

Cited by 159 publications

(189 citation statements)

References 30 publications

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“…We clearly find that the initial simple magnetic field configuration is completely destroyed by the instability, leaving instead a complex non-axisymmetric field with rather small-scale structures very extended in latitude. This is very close to what Braithwaite & Spruit (2006) reported in their models of the instability of a poloidal field in a neutron star. They initiated their models with a different initial field configuration, inspired from the work of Flowers & Ruderman (1977) where the field is uniform in the domain and matches a potential field outside.…”

Section: Characteristics Of the Instabilitysupporting

confidence: 91%

Three-dimensional evolution of magnetic fields in a differentially rotating stellar radiative zone

Jouve

Gastine

Lignières

2015

A&A

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Context. The question of the origin and evolution of magnetic fields in stars possessing a radiative envelope, like the A-type stars, is still regarded as a challenge for stellar physics. Those zones are likely to be differentially rotating, which suggests that strong interactions between differential rotation and magnetic fields could be at play in such regions. Aims. We would like to analyse in detail the evolution of magnetic fields in a differentially rotating stellar radiative zone and the possible presence of magnetic instabilities. Methods. We numerically compute the joint evolution of the magnetic and velocity fields in a 3D spherical shell starting from an initial profile for the poloidal magnetic field and differential rotation. In order to characterise the nature of the magnetic instabilities that may be expected, we use the predictions of a local linear analysis. Results. The poloidal magnetic field is initially wound up by the differential rotation to produce a toroidal field which becomes unstable. We find that different types of instabilities may occur, depending on the balance between the shear strength and the magnetic field intensity. In the particular setup studied here where the differential rotation is dominant, the instability is not of the Tayler-type but is the magneto-rotational instability. The growth rate of the instability depends mainly on the initial rotation rate, while the background state typically oscillates over a poloidal Alfvén time. We thus find that the axisymmetric magnetic configuration is strongly modified by the instability only if the ratio between the poloidal Alfvén frequency and the rotation rate is sufficiently small. An enhanced transport of angular momentum is found in the most unstable cases: the typical time to flatten the rotation profile is then much faster than the decay time associated with the phase-mixing mechanism, which also occurs in the stable cases. When the instability saturates before reaching a significant amplitude, the magnetic configuration relaxes into a stable axisymmetric equilibrium formed by several twisted tori. Conclusions. We conclude that the magneto-rotational instability is always favoured (over the Tayler instability) in unstratified spherical shells when an initial poloidal field is sheared by a sufficiently strong cylindrical differential rotation. A possible application to the magnetic desert observed among A stars is given. We argue that the dichotomy between stars exhibiting strong axisymmetric fields (Ap stars) and those harbouring a sub-Gauss magnetism could be linked to the threshold for the instability.

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Section: Characteristics Of the Instabilitysupporting

confidence: 91%

Three-dimensional evolution of magnetic fields in a differentially rotating stellar radiative zone

Jouve

Gastine

Lignières

2015

A&A

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“…However, this timescale assumes that the dynamo is at saturation, whereas in reality it may take rather longer than this timescale just to reach saturation. This should, in any case, happen faster than τ KH , so this mechanism should dissipate shear energy on a timescale shorter than the thermal timescale on which pre-MS stars evolve (see Braithwaite & Spruit 2014). Consequently, we can expect it to work continuously and efficiently as the convective envelope retreats outward and to keep the radiative zones in approximately solid-body rotation at all times.…”

Section: X-rays From a Shear Dynamomentioning

confidence: 99%

Burn Out or Fade Away? On the X-Ray and Magnetic Death of Intermediate Mass Stars

Drake

Braithwaite²,

Kashyap

et al. 2014

ApJ

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The nature of the mechanisms apparently driving X-rays from intermediate mass stars lacking strong convection zones or massive winds remains poorly understood, and the possible role of hidden, lower mass close companions is still unclear. A 20 ks Chandra HRC-I observation of HR 4796A, an 8 Myr old main sequence A0 star devoid of close stellar companions, has been used to search for a signature or remnant of magnetic activity from the Herbig Ae phase. X-rays were not detected and the X-ray luminosity upper limit was L X 1.3 × 10 27 erg s −1 . The result is discussed in the context of various scenarios for generating magnetic activity, including rotational shear and subsurface convection. A dynamo driven by natal differential rotation is unlikely to produce observable X rays, chiefly because of the difficulty in getting the dissipated energy up to the surface of the star. A subsurface convection layer produced by the ionization of helium could host a dynamo that should be effective throughout the main sequence but can only produce X-ray luminosities of the order 10 25 erg s −1 . This luminosity lies only moderately below the current detection limit for Vega. Our study supports the idea that X-ray production in Herbig Ae/Be stars is linked largely to the accretion process rather than the properties of the underlying star, and that early A stars generally decline in X-ray luminosity at least 100,000 fold in only a few million years.

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“…While there are several indications that the large-scale, external field can be reasonably assumed to be dipolar, with a moderate amount of twist in magnetars, the different mechanisms proposed for the generation of the internal field in the earlier phases of the NS life (differential rotation, dynamo, magneto-rotational instability) most likely give rise to both toroidal and poloidal components (e.g., Geppert et al 2004, 2006 andreferences therein). The presence of a toroidal field, roughly in equipartition with the poloidal one, is also required by general stability arguments (e.g., Braithwaite & Spruit 2006 and references therein). A further complication comes from the present poor knowledge of where the internal field resides.…”

Section: Magneto-rotational Evolutionmentioning

confidence: 99%

Is SGR 0418+5729 Indeed a Waning Magnetar?

Turolla

Zane

Pons

et al. 2011

ApJ

110

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SGR 0418+5729 is a transient soft gamma-ray repeater which underwent a major outburst in 2009 June, during which the emission of short bursts was observed. Its properties appeared quite typical of other sources of the same class until long-term X-ray monitoring failed to detect any period derivative. The present upper limit onṖ implies that the surface dipole field is B p 7.5 × 10 12 G, well below those measured in other soft gamma-ray repeaters (SGRs) and in the Anomalous X-ray Pulsars (AXPs), a group of similar sources. Both SGRs and AXPs are currently believed to be powered by ultra-magnetized neutron stars (magnetars, B p ≈ 10 14 -10 15 G). SGR 0418+5729 hardly seems to fit in such a picture. We show that the magneto-rotational properties of SGR 0418+5729 can be reproduced if this is an aged magnetar, ≈1 Myr old, which experienced substantial field decay. The large initial toroidal component of the internal field required to match the observed properties of SGR 0418+5729 ensures that crustal fractures, and hence bursting activity, can still occur at the present time. The thermal spectrum observed during the outburst decay is compatible with the predictions of a resonant Compton scattering model (as in other SGRs/AXPs) if the field is low and the magnetospheric twist is moderate.

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Evolution of the magnetic field in magnetars

Cited by 159 publications

References 30 publications

Three-dimensional evolution of magnetic fields in a differentially rotating stellar radiative zone

Three-dimensional evolution of magnetic fields in a differentially rotating stellar radiative zone

Burn Out or Fade Away? On the X-Ray and Magnetic Death of Intermediate Mass Stars

Is SGR 0418+5729 Indeed a Waning Magnetar?

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