The source of magnetic fields in (neutron-) stars

AbstractSome arguments, none entirely conclusive, are reviewed about the origin of magnetic fields in neutron stars, with emphasis of processes during and following core collapse in supernovae. Possible origins of the magnetic fields of neutron stars include inheritance from the main sequence progenitor and dynamo action at some stage of evolution of progenitor. Inheritance is not sufficient to explain the fields of magnetars. Energetic considerations point to differential rotation in the final stages of core collapse process as the most likely source of field generation, at least for magnetars. A runaway phase of exponential growth is needed to achieve sufficient field amplification during relevant phase of core collapse; it can probably be provided by a some form of magnetorotational instability. Once formed in core collapse, the field is in danger of decaying again by magnetic instabilities. The evolution of a magnetic field in a newly formed neutron star is discussed, with emphasis on the existence of stable equilibrium configurations as end products of this evolution, and the role of magnetic helicity in their existence. A particularly puzzling problem is the large range of field strengths observed in neutron stars (as well as in A stars and white dwarfs). It implies that a single, deterministic process is insufficient to explain the origin of the magnetic fields in these stars.

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How to form a millisecond magnetar? Magnetic field amplification in protoneutron stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317004732 ◽

2017 ◽

Vol 12 (S331) ◽

pp. 119-124 ◽

Cited By ~ 1

Author(s):

Jérôme Guilet ◽

Ewald Müller ◽

Hans-Thomas Janka ◽

Tomasz Rembiasz ◽

Martin Obergaulinger ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Gamma Ray ◽

Core Collapse ◽

Magnetorotational Instability ◽

Strong Magnetic Fields ◽

Physical Conditions ◽

Field Amplification ◽

Magnetic Prandtl Number ◽

Protoneutron Stars

AbstractExtremely strong magnetic fields of the order of 1015G are required to explain the properties of magnetars, the most magnetic neutron stars. Such a strong magnetic field is expected to play an important role for the dynamics of core-collapse supernovae, and in the presence of rapid rotation may power superluminous supernovae and hypernovae associated to long gamma-ray bursts. The origin of these strong magnetic fields remains, however, obscure and most likely requires an amplification over many orders of magnitude in the protoneutron star. One of the most promising agents is the magnetorotational instability (MRI), which can in principle amplify exponentially fast a weak initial magnetic field to a dynamically relevant strength. We describe our current understanding of the MRI in protoneutron stars and show recent results on its dependence on physical conditions specific to protoneutron stars such as neutrino radiation, strong buoyancy effects and large magnetic Prandtl number.

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On the role of rotation in the generation of magnetic fields by fluid motions

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1982.0082 ◽

1982 ◽

Vol 306 (1492) ◽

pp. 223-234 ◽

Cited By ~ 20

Keyword(s):

Magnetic Fields ◽

Axial Symmetry ◽

Fluid Motion ◽

Liquid Core ◽

Flow Speed ◽

Magnetic Reynolds Number ◽

Dynamo Action ◽

Coriolis Forces ◽

Lorentz Forces

It is generally accepted that the magnetic fields of planets and stars are produced by the self-exciting dynamo process (first proposed by Larmor) and that observed near-alignments of magnetic dipole axes with rotation axes are due to the influence of Coriolis forces on underlying fluid motions. The detailed role of rotation in the generation of cosmical magnetic fields has yet to be elucidated but useful insight can be obtained from general considerations of the governing magnetohydrodynamic equations. A magnetic field B cannot be maintained or amplified by fluid motion u against the effects of ohmic decay unless (a) the magnetic Reynolds number R = ULno is sufficiently large, and (b) the patterns of B and u are sufficiently complicated (where U is a characteristic flow speed, a characteristic length and J and o are typical values of the magnetic permeability and electrical conductivity respectively). Axisymmetric magnetic fields will always decay (a result that suggests that palaeomagnetic and archaeomagnetic data might show evidence that departures from axial symmetry in the geomagnetic field are systematically less during the decay phase of a polarity ‘ reversal ’ or * excursion ’ than during the recovery phase). Dynamo action is stimulated by Coriolis forces, which promote departures from axial symmetry in the pattern of uwhen B is weak, and is opposed by Lorentz forces, which increase in influence as B grows in strength. If equilibrium is attained when Coriolis and Lorentz forces are roughly equal in magnitude then the system becomes ‘ magnetostrophic' and the strengths of the internal and external parts of the field, and respectively, satisfy B i < B 8 R 1/2 and B e < B 8 R -1/2 if B 8 = UL~1 *X)/c f)i » (p being the mean density of the fluid and Q the angular speed of rotation). The slow and dispersive ‘magnetohydrodynamic inertial wave’ with a frequency that depends on the square of the Alfven speed [B]/(up) 1/2 and inversely on Q exemplifies magnetostrophic flow. Such waves probably occur in the electrically conducting fluid interiors of planets and stars, where they play an important role in the generation of magnetic fields as well as in other processes, such as the topographic coupling between the Earth’s liquid core and solid mantle.

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Updates of the nuclear equation of state for core-collapse supernovae and neutron stars: effects of 3-body forces, QCD, and magnetic fields

Journal of Physics Conference Series ◽

10.1088/1742-6596/445/1/012023 ◽

2013 ◽

Vol 445 ◽

pp. 012023

Author(s):

G J Mathews ◽

M Meixner ◽

J P Olson ◽

I-S Suh ◽

T Kajino ◽

...

Keyword(s):

Equation Of State ◽

Magnetic Fields ◽

Neutron Stars ◽

Core Collapse ◽

Nuclear Equation Of State ◽

Body Forces

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Very young neutron stars and millisecond pulsars: the role of the accretion

Proceedings of the International Astronomical Union ◽

10.1017/s174392131201976x ◽

2012 ◽

Vol 8 (S290) ◽

pp. 231-232

Author(s):

Alexander F. Kholtygin ◽

Andrei P. Igoshev

Keyword(s):

Magnetic Fields ◽

Neutron Stars ◽

Binary Systems ◽

Compact Objects ◽

Millisecond Pulsars ◽

Know How ◽

Central Compact Objects ◽

Low Magnetic Field ◽

Age Determinations

AbstractWe consider the evolution of the very young neutron stars (NS) with moderate and low magnetic field values around 1E8 G to know how large is the share of the these objects among the those attributed as the millisecond pulsars (MSP). To exclude the contamination of accreted NS and young NS with moderate magnetic fields we study the observational evidences of the accretion on NS in the binary systems and different methods of age determinations. It was concluded that only central compact objects are appropriate candidates for NSs with small initial magnetic fields.

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Galactic and Metagalactic Magnetic Fields

Symposium - International Astronomical Union ◽

10.1017/s0074180900006021 ◽

1972 ◽

Vol 44 ◽

pp. 520-525

Author(s):

K. Brecher

Keyword(s):

Magnetic Field ◽

Cosmic Rays ◽

Magnetic Fields ◽

Neutron Stars ◽

Galactic Nuclei ◽

Direct Result ◽

Field Amplification ◽

Recent Origin ◽

Intergalactic Magnetic Field ◽

Rotating Objects

All of the possible sources of primeval magnetic fields are examined and found to be inadequate to account for the observed galactic B-fields (if the magnetic fields in galaxies are the direct result of the condensation of a magnetized metagalactic medium). Field amplification in dense rotating objects (perhaps neutron stars or galactic nuclei) may be responsible for the bulk of such fields. Most of the (hypothetical) intergalactic B-field then originates in galaxies and is dragged along with the cosmic rays escaping from them. Limits are placed on the value of any present intergalactic magnetic field of either primordial or ‘recent’ origin.

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Magnetic field amplification in turbulent astrophysical plasmas

Journal of Plasma Physics ◽

10.1017/s0022377816001069 ◽

2016 ◽

Vol 82 (6) ◽

Cited By ~ 39

Author(s):

Christoph Federrath

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Formation Rate ◽

Density Fluctuations ◽

Accretion Discs ◽

Theoretical Modelling ◽

Dynamo Action ◽

Astrophysical Plasmas ◽

Field Amplification ◽

Prandtl Numbers

Magnetic fields play an important role in astrophysical accretion discs and in the interstellar and intergalactic medium. They drive jets, suppress fragmentation in star-forming clouds and can have a significant impact on the accretion rate of stars. However, the exact amplification mechanisms of cosmic magnetic fields remain relatively poorly understood. Here, I start by reviewing recent advances in the numerical and theoretical modelling of the turbulent dynamo, which may explain the origin of galactic and intergalactic magnetic fields. While dynamo action was previously investigated in great detail for incompressible plasmas, I here place particular emphasis on highly compressible astrophysical plasmas, which are characterised by strong density fluctuations and shocks, such as the interstellar medium. I find that dynamo action works not only in subsonic plasmas, but also in highly supersonic, compressible plasmas, as well as for low and high magnetic Prandtl numbers. I further present new numerical simulations from which I determine the growth of the turbulent (un-ordered) magnetic field component ($B_{turb}$) in the presence of weak and strong guide fields ($B_{0}$). I vary $B_{0}$ over five orders of magnitude and find that the dependence of $B_{turb}$ on $B_{0}$ is relatively weak, and can be explained with a simple theoretical model in which the turbulence provides the energy to amplify $B_{turb}$. Finally, I discuss some important implications of magnetic fields for the structure of accretion discs, the launching of jets and the star-formation rate of interstellar clouds.

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MAGNETO-PLASMA PROCESSES IN RELATIVISTIC ASTROPHYSICS: MODERN DEVELOPMENTS

International Journal of Modern Physics D ◽

10.1142/s0218271813300164 ◽

2013 ◽

Vol 22 (07) ◽

pp. 1330016

Author(s):

OLEG YU. TSUPKO

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Gravitational Lensing ◽

Crucial Role ◽

Compact Objects ◽

Relativistic Astrophysics ◽

Core Collapse ◽

Core Collapse Supernova ◽

Supernova Explosions

This contribution is a review of some talks presented at the session "Magneto-Plasma Processes in Relativistic Astrophysics" of the Thirteenth Marcel Grossmann Meeting MG13. We discuss the modern developments of relativistic astrophysics, connected with presence of plasma and magnetic fields. The influence of magneto-plasma processes on the structure of the compact objects and accretion processes is considered. We also discuss a crucial role of magnetic field for the mechanism of core-collapse supernova explosions. Gravitational lensing in plasma is also considered.

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SGRs and AXPs: Evidence for Delayed Amplification of Magnetic Field after Neutron Star Formation?

Advances in Astronomy ◽

10.1155/2009/306821 ◽

2009 ◽

Vol 2009 ◽

pp. 1-8 ◽

Cited By ~ 7

Author(s):

Denis Leahy ◽

Rachid Ouyed

Keyword(s):

Magnetic Field ◽

Neutron Star ◽

Magnetic Fields ◽

Neutron Stars ◽

Birth Rate ◽

Massive Stars ◽

Robust Estimator ◽

High Magnetic Fields ◽

Field Amplification ◽

Kolmogorov Smirnov

We present new analysis of the birth rate of AXPs and SGRS and their associated SNRs. Using Kolmogorov-Smirnov statistics together with parametric fits based on a robust estimator, we find a birth rate of ∼1/(1000 years) for AXPs/SGRs and their associated SNRs. These high rates suggest that all massive stars (greater than ∼(23–32)M⊙) give rise to remnants with magnetar-like fields. Observations indicate a limited fraction of high magnetic fields in these progenitors; thus our study is suggestive of magnetic field amplification. Dynamo mechanisms during the birth of the neutron stars require spin rates much faster than either observations or theory indicate. We propose that massive stars produce neutron stars with normal (∼1012 G) magnetic fields, which are then amplified to1014-1015 G after a delay of hundreds of years. The amplification is speculated to be a consequence of color ferromagnetism and to occur with a delay after the neutron star core reaches quark deconfinement density (i.e., the quark-nova scenario). The delayed amplification allows one to interpret simultaneously the high birth rate and high magnetic fields of AXPs/SGRs and their link to massive stars.

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Convection and dynamo action in B stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311017790 ◽

2010 ◽

Vol 6 (S271) ◽

pp. 361-362 ◽

Cited By ~ 3

Author(s):

Kyle C. Augustson ◽

Allan S. Brun ◽

Juri Toomre

Keyword(s):

Magnetic Fields ◽

Main Sequence ◽

Magnetic Energy ◽

Magnetic Polarity ◽

Dynamo Action ◽

B Stars ◽

Core Convection ◽

Dependent Behavior ◽

And Shifts

AbstractMain-sequence massive stars possess convective cores that likely harbor strong dynamo action. To assess the role of core convection in building magnetic fields within these stars, we employ the 3-D anelastic spherical harmonic (ASH) code to model turbulent dynamics within a 10 M⊙ main-sequence (MS) B-type star rotating at 4 Ω⊙. We find that strong (900 kG) magnetic fields arise within the turbulence of the core and penetrate into the stably stratified radiative zone. These fields exhibit complex, time-dependent behavior including reversals in magnetic polarity and shifts between which hemisphere dominates the total magnetic energy.

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