Low-magnetic-field magnetars

TL;DR

This paper reviews low-magnetic-field magnetars, discussing their observational properties and implications for the magnetar model, especially considering recent discoveries of magnetars with magnetic fields comparable to ordinary pulsars.

Contribution

It provides a comprehensive summary of low-magnetic-field magnetars and analyzes their properties within the context of existing magnetar theories and alternative models.

Findings

Discovery of magnetars with magnetic fields similar to radio pulsars.

Low-field magnetars can exhibit typical magnetar activity.

Challenges to the standard high-field magnetar paradigm.

Abstract

It is now widely accepted that soft gamma repeaters and anomalous X-ray pulsars are the observational manifestations of magnetars, i.e. sources powered by their own magnetic energy. This view was supported by the fact that these `magnetar candidates' exhibited, without exception, a surface dipole magnetic field (as inferred from the spin-down rate) in excess of the electron critical field (~4.4E+13 G). The recent discovery of fully-qualified magnetars, SGR 0418+5729 and Swift J1822.3-1606, with dipole magnetic field well in the range of ordinary radio pulsars posed a challenge to the standard picture, showing that a very strong field is not necessary for the onset of magnetar activity (chiefly bursts and outbursts). Here we summarize the observational status of the low-magnetic-field magnetars and discuss their properties in the context of the mainstream magnetar model and its main alternatives.