A low-magnetic-field Soft Gamma Repeater

TL;DR

This paper reports the discovery of a soft gamma repeater with a low magnetic field, challenging the idea that high magnetic fields are necessary for magnetar activity, and suggesting a broader magnetar population.

Contribution

It presents the first observation of a magnetar-like object with a magnetic field comparable to ordinary pulsars, expanding the understanding of magnetar diversity.

Findings

The magnetic field of SGR 0418+5729 is below 7.5x10^{12} Gauss.

Magnetar-like activity can occur at lower magnetic fields than previously thought.

The magnetar population may include objects with a wider range of magnetic fields.

Abstract

Soft gamma repeaters and anomalous x-ray pulsars form a rapidly increasing group of x-ray sources exhibiting sporadic emission of short bursts. They are believed to be magnetars, i.e. neutron stars powered by extreme magnetic fields, B~10^{14}-10^{15} Gauss. We report on a soft gamma repeater with low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of magnetars. X-ray observations show that its dipolar magnetic field cannot be greater than 7.5x10^{12} Gauss, 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 B-field strengths, ages and evolutionary stages than observed so far.