Magnetic field distribution in magnetars

TL;DR

This paper develops a universal magnetic field profile for magnetized neutron stars using axisymmetric simulations, improving modeling accuracy and providing a practical tool for nuclear physics research.

Contribution

It introduces a simple polynomial profile for the magnetic field's monopolar component applicable to various neutron star configurations, enhancing existing modeling approaches.

Findings

The magnetic field profile fits well across different star models.

Modified TOV equations with the profile yield better mass-radius results.

Spherical symmetry assumptions are less accurate for realistic models.

Abstract

Using an axisymmetric numerical code, we perform an extensive study of the magnetic field configurations in non-rotating neutron stars, varying the mass, magnetic field strength and the equation of state. We find that the monopolar (spherically symmetric) part of the norm of the magnetic field can be described by a single profile, that we fit by a simple eighth-order polynomial, as a function of the star's radius. This new generic profile applies remarkably well to all magnetized neutron star configurations built on hadronic equations of state. We then apply this profile to build magnetized neutron stars in spherical symmetry, using a modified Tolman-Oppenheimer-Volkov (TOV) system of equations. This new formalism produces slightly better results in terms of mass-radius diagrams than previous attempts to add magnetic terms to these equations. However, we show that such approaches are less accurate than usual, non-magnetized TOV models, and that consistent models must depart from spherical symmetry. Thus, our `universal' magnetic field profile is intended to serve as a tool for nuclear physicists to obtain estimates of magnetic field inside neutron stars, as a function of radial depth, in order to deduce its influence on composition and related properties. It possesses the advantage of being based on magnetic field distributions from realistic self-consistent computations, which are solutions of Maxwell's equations.