Ambipolar decay of magnetic field in magnetars and the observed magnetar activities

TL;DR

This paper investigates how ambipolar diffusion causes magnetic field decay in magnetars, providing a detailed model that aligns with observed magnetar activity and decay timescales.

Contribution

It presents a comprehensive method to calculate ambipolar velocity and decay rates in neutron star cores without approximations, enhancing understanding of magnetic field evolution in magnetars.

Findings

Ambipolar diffusion significantly contributes to magnetic field decay in magnetars.

The decay timescale derived matches observed magnetar activity periods.

The model offers a realistic depiction of magnetic field evolution in neutron star cores.

Abstract

Magnetars are comparatively young neutron stars with ultra-strong surface magnetic field in the range $10^{14-16}$ G. The old neutron stars have surface magnetic field some what less $\sim 10^8$ G which clearly indicates the decay of field with time. One possible way of magnetic field decay is by ambipolar diffusion. We describe the general procedure to solve for the ambipolar velocity inside the star core without any approximation. With a realistic model of neutron star we determine the ambipolar velocity configuration inside the neutron star core and hence find the ambipolar decay rate and time scale which is consistent with the magnetar observations.