A Low-Magnetic-Field Soft Gamma Repeater

Odd Magnetar Magnetars are neutron stars that are widely thought to be powered by extremely high magnetic fields. Using data from three different x-ray observatories, Rea et al. (p. 944, published online 14 October) show that a previously known magnetar has a magnetic field that is much smaller than those of other magnetars. A strong magnetic field is thus not a necessary requirement for a neutron star to show magnetar-like behavior, implying that this population is wider than was previously thought. A neutron star lacking a strong magnetic field can still emit short bursts of high-energy radiation. Soft gamma repeaters (SGRs) 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, that is, neutron stars powered by extreme magnetic fields, B ~ 1014 to 1015 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 7.5 × 1012 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.