A Low-Magnetic-Field Soft Gamma Repeater
Tiengo, A.; Götz, D.; Mereghetti, S.; Stella, L.; Kouveliotou, C.; Rea, N.; Esposito, P.; Israel, G. L.; Zane, S.; Turolla, R.; Göğüş, E.
Spain, Italy, United Kingdom, France, Turkey, United States
Abstract
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.