From proto-neutron star dynamo to low-field magnetars

TL;DR

This paper demonstrates through numerical simulations that low-field magnetars can form via a Tayler--Spruit dynamo in proto-neutron stars, explaining their magnetic properties and burst activity.

Contribution

It introduces the first direct magneto-thermal evolution simulations showing low-field magnetar formation from a dynamo-generated magnetic field.

Findings

Low-field magnetars have weak dipolar fields but strong small-scale fields.

Simulations reproduce magnetar-like X-ray bursts due to crustal failures.

Two formation channels for classical and low-field magnetars are proposed.

Abstract

Low-field magnetars have dipolar magnetic fields that are 10-100 times weaker than the threshold, $B \gtrsim 10^{14}$ G, used to define classical magnetars, yet they produce similar X-ray bursts and outbursts. Using the first direct numerical simulations of magneto-thermal evolution starting from a dynamo-generated magnetic field, we show that the low-field magnetars can be produced as a result of a Tayler--Spruit dynamo inside the proto-neutron star. We find that these simulations naturally explain key characteristics of low-field magnetars: (1) weak ($\lesssim 10^{13}$ G) dipolar magnetic fields, (2) strong small-scale fields, and (3) magnetically induced crustal failures producing X-ray bursts. These findings suggest two distinct formation channels for classical and low-field magnetars, potentially linked to different dynamo mechanisms.