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Validating and improving two-fluid simulations of the magnetic field evolution in neutron star cores
High Energy Astrophysical Phenomena · Submitted 14 Mar 2025
abstract compiled · 2026-09-23 · no language-model review
The authors' main claim We also find that, in the long term, the star evolves towards a barotropic ``Grad-Shafranov equilibrium,'' where the magnetic force is fully balanced by charged particle fluid forces.
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The abstract, sentence by sentence
This paper addresses the evolution of an axially symmetric magnetic field in the core of a neutron star. The matter in the core is modeled as a system of two fluids, namely neutrons and charged particles, with slightly different velocity fields, controlled by their mutual collisional friction. This problem was addressed in our previous work through the so-called ``fictitious friction'' approach. We study the validity of our previous work and improve it by comparing the fictitious friction approach to alternatives, making approximations that allow it to be applied to arbitrary magnetic field strengths and using realistic equations of state. We assume the neutron star crust to be perfectly resistive, so its magnetic field reacts instantaneously to changes in the core, in which we neglect the effects of Cooper pairing. We explore different approaches to solve the equations to obtain the velocities and chemical potential perturbations induced by a given, fixed magnetic field configuration in the core. We also present a new version of our code to perform time-evolving simulations and discuss the results obtained with it. Our calculations without fictitious friction further confirm that bulk velocity is generally much greater than ambipolar velocity, leading to faster evolution. These findings align with those with fictitious friction, validating this approach. We also find that, in the long term, the star evolves towards a barotropic ``Grad-Shafranov equilibrium,'' where the magnetic force is fully balanced by charged particle fluid forces. Qualitatively, the evolution and the final equilibrium are independent of the magnetic field strength $B$ and the equation of state considered. The timescale to reach this equilibrium is proportional to $B^{-2}$ and becomes shorter for equations of state with a smaller gradient of the ratio between the densities of protons and neutrons.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | This paper addresses the evolution of an axially symmetric magnetic field in the core of a neutron star. | background 0.82 | 0.37 | 0.02 | asserted 0.94 |
| s1 | The matter in the core is modeled as a system of two fluids, namely neutrons and charged particles, with slightly different velocity fields, controlled by their mutual collisional friction. | method 0.97 | 0.56 | 0.02 | asserted 1.00 |
| s2 | This problem was addressed in our previous work through the so-called ``fictitious friction'' approach. | background 0.97 | 0.43 | 0.02 | asserted 1.00 |
| s3 | We study the validity of our previous work and improve it by comparing the fictitious friction approach to alternatives, making approximations that allow it to be applied to arbitrary magnetic field strengths and using realistic equations of state. | contribution 0.99 | 0.91 | 0.07 | asserted 0.94 |
| s4 | We assume the neutron star crust to be perfectly resistive, so its magnetic field reacts instantaneously to changes in the core, in which we neglect the effects of Cooper pairing. | method 0.62 | 0.57 | 0.02 | asserted 1.00 |
| s5 | We explore different approaches to solve the equations to obtain the velocities and chemical potential perturbations induced by a given, fixed magnetic field configuration in the core. | method 0.88 | 0.91 | 0.02 | asserted 0.87 |
| s6 | We also present a new version of our code to perform time-evolving simulations and discuss the results obtained with it. | contribution 0.70 | 0.94 | 0.03 | asserted 0.99 |
| s7 | Our calculations without fictitious friction further confirm that bulk velocity is generally much greater than ambipolar velocity, leading to faster evolution. | result 0.99 | 0.93 | 0.16 | asserted 0.99 |
| s8 | These findings align with those with fictitious friction, validating this approach. | interpretation 0.69 | 0.82 | 0.18 | asserted 0.98 |
| s9 | We also find that, in the long term, the star evolves towards a barotropic ``Grad-Shafranov equilibrium,'' where the magnetic force is fully balanced by charged particle fluid forces. | result 1.00 | 0.95 | 0.30 | asserted 1.00 |
| s10 | Qualitatively, the evolution and the final equilibrium are independent of the magnetic field strength $B$ and the equation of state considered. | result 0.98 | 0.89 | 0.24 | asserted 0.99 |
| s11 | The timescale to reach this equilibrium is proportional to $B^{-2}$ and becomes shorter for equations of state with a smaller gradient of the ratio between the densities of protons and neutrons. | result 1.00 | 0.93 | 0.20 | asserted 1.00 |
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The paper
Original abstract
This paper addresses the evolution of an axially symmetric magnetic field in the core of a neutron star. The matter in the core is modeled as a system of two fluids, namely neutrons and charged particles, with slightly different velocity fields, controlled by their mutual collisional friction. This problem was addressed in our previous work through the so-called ``fictitious friction'' approach. We study the validity of our previous work and improve it by comparing the fictitious friction approach to alternatives, making approximations that allow it to be applied to arbitrary magnetic field strengths and using realistic equations of state. We assume the neutron star crust to be perfectly resistive, so its magnetic field reacts instantaneously to changes in the core, in which we neglect the effects of Cooper pairing. We explore different approaches to solve the equations to obtain the velocities and chemical potential perturbations induced by a given, fixed magnetic field configuration in the core. We also present a new version of our code to perform time-evolving simulations and discuss the results obtained with it. Our calculations without fictitious friction further confirm that bulk velocity is generally much greater than ambipolar velocity, leading to faster evolution. These findings align with those with fictitious friction, validating this approach. We also find that, in the long term, the star evolves towards a barotropic ``Grad-Shafranov equilibrium,'' where the magnetic force is fully balanced by charged particle fluid forces. Qualitatively, the evolution and the final equilibrium are independent of the magnetic field strength $B$ and the equation of state considered. The timescale to reach this equilibrium is proportional to $B^{-2}$ and becomes shorter for equations of state with a smaller gradient of the ratio between the densities of protons and neutrons.
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How the literature uses it
2 arXiv papers cite this in 8 sentences
Papers citing it · 6 sentences without a confident label
Uses its method or tool 1 paper
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To avoid these difficulties, we use the artificial friction method originally implemented for NS cores by Refs. [ref] in one-dimensional simulations, then in axial symmetry by Ref. [ref] and [ref] , and recently validated by Ref. [ref] .
Magnetothermal evolution of neutron star cores in the `weak-coupling' regime: implications of ambipolar diffusion for the quiescent X-ray luminosity of magnetars · Artificial friction method · 0.99
Cites it as support 1 paper
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As first shown by Ref. [ref] and [ref] , and later corroborated with detailed numerical simulations by Refs. [ref] , the bulk neutron velocity is larger than the ambipolar velocity by a factor $v_n/v_ad ∼10-100$ depending on the magnetic field geometry and EoS in the core.
Magnetothermal evolution of neutron star cores in the `weak-coupling' regime: implications of ambipolar diffusion for the quiescent X-ray luminosity of magnetars · Timescale · 0.96
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