Magnetic field evolution in neutron stars

Abstract

Neutron stars contain persistent, ordered magnetic fields that are the strongest known in the Universe. However, their magnetic fluxes are similar to those in magnetic A and B stars and white dwarfs, suggesting that flux conservation during gravitational collapse may play an important role in establishing the field, although it might also be modified substantially by early convection, differential rotation, and magnetic instabilities. The equilibrium field configuration, established within hours (at most) of the formation of the star, is likely to be roughly axisymmetric, involving both poloidal and toroidal components. The stable stratification of the neutron star matter (due to its radial composition gradient) probably plays a crucial role in holding this magnetic structure inside the star. The field can evolve on long time scales by processes that overcome the stable stratification, such as weak interactions changing the relative abundances and ambipolar diffusion of charged particles with respect to neutrons. These processes become more effective for stronger magnetic fields, thus naturally explaining the magnetic energy dissipation expected in magnetars, at the same time as the longer-lived, weaker fields in classical and millisecond pulsars.Comment: To appear in Astronomische Nachrichten (Astronomical Notes) as part of the Proceedings of the 5th Potsdam Thinkshop, "Meridional Circulation, Differential Rotation, Solar and Stellar Activity", held 2007 June 24-29. 5 pages, no figure

On non-axisymmetric magnetic equilibria in stars

“…Hall drift and buoyant rise provide natural explanations for this as they scale with the field strength, as B and B 2 , respectively. Buoyancy of flux tubes in NSs depends mainly on the composition gradient, the difference with WDs being that not only can chemical elements diffuse into or out of a flux tube but also that the chemical elements can reach equilibrium via the neutrino‐emitting reaction n ⇔ p + e (Reisenegger 2007). The speed of the latter does not depend on the length‐scale, so fields with narrow flux tubes would evolve no faster than simpler fields if this were the dominant mechanism.…”

“…In an axisymmetric equilibrium, the buoyant-rising effect of thermal diffusion is resisted by the tension of the toroidal component. As Reisenegger (2007) points out, this is likely to be more important when the torus is buried deep inside the star, which could explain why magnetic diffusion may be more important in deeply buried fields. 5 As axisymmetric equilibrium with a flatter radial energy profile diffuses outwards, it experiences the same loss of toroidal flux as the non-axisymmetric equilibria.…”

“…In an axisymmetric equilibrium, the buoyant‐rising effect of thermal diffusion is resisted by the tension of the toroidal component. As Reisenegger (2007) points out, this is likely to be more important when the torus is buried deep inside the star, which could explain why magnetic diffusion may be more important in deeply buried fields 5…”

First detection of a magnetic field in the fast rotating runaway Oe star ζ Ophiuchi

“…In each of these classes there is a wide distribution of magnetic field strengths, but the distribution of magnetic fluxes appears to be similar in each class, with maxima of Φ max = πR 2 B ∼ 10 27−28 G cm 2 (Reisenegger 2001; Ferrario & Wickramasinghe 2005), arguing for a fossil field whose flux is conserved along the path of stellar evolution. According to Reisenegger (2009) the magnetic fluxes have possibly been generated on or even before the main-sequence stage and then inherited by the compact remnants.…”

On Magnetic Fields in Rotating Nuclear Matter Cores of Stellar Dimensions

“…However the origin of the magnetic field in the neutron stars is not fully understood, so far. Nevertheless in the literature one may find various hypotheses explaining the formation of the magnetic field 1,2,3,4,5,6,7 . The simplest hypothesis to explain the presence of the strong fields observed in neutron stars is the conservation of the magnetic flux already present in the progenitor stars during the gravitational collapse.…”