An investigation of magnetic field distortions in accretion discs around neutron stars - II. Analysis of the toroidal field component

Abstract

Millisecond pulsars are believed to be old pulsars spun up by a surrounding accretion disc. Magnetic fields are thought to play a leading role in this, both by determining the location of the inner edge of the disc and by exerting an additional torque on the star (as a result of the interaction between the stellar magnetic field and the disc plasma motion, which creates a toroidal component B_phi). In some well-known analytic models, developed in the 1980s, the B_phi profile was taken to be proportional to the relative angular velocity between the disc plasma and the neutron star, multiplied by a vertical dipolar field. The present work stands in the line of improving those models, suggesting a new profile for B. In a previous paper, we discussed the poloidal component of the magnetic field and here we consider the toroidal component, again making the kinematic approximation and looking for steady solutions of the induction equation for axisymmetric models. The poloidal magnetic field is not assumed to be dipolar and the poloidal velocity field is not taken to be zero everywhere. We also do not use the thin disc approximation to simplify the induction equation but instead solve it numerically in full 2D. The profile obtained in the earlier analytic models is shown to have very limited validity and a more general semi-analytic solution is proposed.

arXiv commentPublished in A&A - 13 pages, 17 figures and one table - Changes with respect to previous version: added comment on magnetic Prandtl number; minor modifications (added comment on mean field induction equation, removed confusion about the names of some variables, updated references, merged two figures, removed references from abstract)

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Cited by in corpus (5)

• GRMHD simulations of accreting neutron stars I: nonrotating dipoles
• Magnetic field structure and torque in accretion discs around millisecond pulsars
• Magnetically threaded accretion disks in resistive magnetohydrodynamic simulations and asymptotic expansion
• The Structure of a Quasi-Keplerian Accretion Disk around Magnetized Stars
• Magnetic fields in Accretion Discs around Neutron Stars - Consequences for the change of spin