Is SGR 0418+5729 indeed a waning magnetar ?

TL;DR

This paper proposes that SGR 0418+5729 is an aged magnetar with a decayed magnetic field, explaining its atypically low dipole field and persistent bursting activity through a model of magnetic field decay over approximately one million years.

Contribution

It introduces a magneto-rotational evolution model showing SGR 0418+5729 as an old magnetar with significant magnetic decay, challenging the traditional view of magnetars as ultra-magnetized neutron stars.

Findings

SGR 0418+5729's properties can be explained by magnetic field decay over 1 million years.

The source's thermal spectrum aligns with a low magnetic field and moderate magnetospheric twist.

Crustal fractures and bursting activity are possible despite the low current magnetic field.

Abstract

SGR 0418+5729 is a transient Soft Gamma-ray Repeater which underwent a major outburst in June 2009, during which the emission of short bursts was observed. Its properties appeared quite typical of other sources of the same class until long-term X-ray monitoring failed to detect any period derivative. The present upper limit on $\dot P$ implies that the surface dipole field is $B_p\lesssim 7.5\times 10^{12}\ {\rm G}$ (Rea et al 2010), well below those measured in other Soft Gamma-ray Repeaters (SGRs) and in the Anomalous X-ray Pulsars (AXPs), a group of similar sources. Both SGRs and AXPs are currently believed to be powered by ultra-magnetized neutron stars (magnetars, $B_p\approx 10^{14}$--$10^{15}\ {\rm G}$). SGR 0418+5729 hardly seems to fit in such a picture. We show that the magneto-rotational properties of SGR 0418+5729 can be reproduced if this is an aged magnetar, $\approx 1\ {\rm Myr}$ old, which experienced substantial field decay. The large initial toroidal component of the internal field required to match the observed properties of SGR 0418+5729 ensures that crustal fractures, and hence bursting activity, can still occur at present time. The thermal spectrum observed during the outburst decay is compatible with the predictions of a resonant compton scattering model (as in other SGRs/AXPs) if the field is low and the magnetospheric twist moderate.