Magnetic field dissipation in neutron star crusts: from magnetars to isolated neutron stars

Departament de Física Aplicada, Universitat d'Alacant, Ap. Correus 99, 03080 Alacant, Spain e-mail: Jose.A.Pons@ua.es

Received: 12 March 2007
Accepted: 8 April 2007

Abstract

Context.We study the non-linear evolution of magnetic fields in neutron star crusts with special attention to the influence of the Hall drift.

Aims.Our goal is to understand the conditions for fast dissipation due to the Hall term in the induction equation. We study the interplay of Ohmic dissipation and Hall drift in order to find a timescale for the overall crustal field decay.

Methods.We solve the Hall induction equation numerically by means of a hybrid method (spectral in angles but finite differences in the radial coordinate). The microphysical input consists of the most modern available crustal equation of state, composition, and electrical conductivities.

Results.We present the first long-term simulations of the non-linear magnetic field evolution in realistic neutron star crusts with a stratified electron number density and temperature dependent conductivity. We show that Ohmic dissipation influenced by Hall drift takes place in neutron star crusts on a timescale of 10⁶ years. When the initial magnetic field has magnetar strength, the fast Hall drift results in an initial rapid dissipation stage that lasts ~10⁴ years. The interplay of the Hall drift with the temporal variation and spatial gradient of conductivity tends to favor the displacement of toroidal fields toward the inner crust, where stable configurations can last for ~10⁶ years. We show that the thermally emitting, isolated neutron stars, such as the Magnificent Seven, are very likely descendants of neutron stars born as magnetars.