SGR 0418+5729: a low-magnetic-field magnetar

TL;DR

This paper reports the discovery of a magnetar, SGR 0418+5729, with a surprisingly low magnetic field, challenging the idea that high magnetic fields are necessary for magnetar activity and suggesting a broader diversity in magnetar properties.

Contribution

It provides evidence for a magnetar with a low dipolar magnetic field, expanding the known range of magnetic field strengths in magnetars and challenging existing theories.

Findings

Magnetar SGR 0418+5729 has a magnetic field below 8E12 G.

Magnetar activity can occur in objects with magnetic fields similar to radio pulsars.

The magnetar population may be more diverse than previously thought.

Abstract

Soft gamma-ray repeaters and anomalous X-ray pulsars are a small (but growing) group of X-ray sources characterized by the emission of short bursts and by a large variability in their persistent flux. They are believed to be magnetars, i.e. neutron stars powered by extreme magnetic fields 1E14-1E15 G). We found evidence for a magnetar with a low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of soft gamma-ray repeaters. New X-ray observations show that its dipolar magnetic field cannot be greater than 8E12 G, well in the range of ordinary radio pulsars, implying that a high surface dipolar magnetic field is not necessarily required for magnetar-like activity. The magnetar population may thus include objects with a wider range of magnetic-field strengths, ages and evolutionary stages than observed so far.