Soft Gamma Repeaters

Soft gamma-ray repeaters (SGRs) are compact objects undergoing spasmodic instabilities producing X-ray super-Eddington outbursts (sometimes in rapid succession, even 38 events in 350 s as in the case of SGR 1900+14on May 30th 1998. Recently, the accurate positioning of X-ray outbursts from SGR 1806-20 confirmed that some SGRs are within Galactic supernova remnants. Also the remarkable source SGR 0525-66 that produced the `March 5th event' ,showing the first evidence of a $\sim$8 s periodicity following a very intense initial pulse, is positionally coincident with the N49 supernova remnant in the Large Magellanic Cloud. The combination of a relatively long time-scale oscillation (interpreted as rotation of a magnetized compact object) and a lifetime comparable with that of the associated remnant supported a model based on a strongly magnetized ($B \sim 10^{14}-10^{15}$ G) neutron star for SGR 0525-66. The recent detection of 7.47 s pulsations in the persistent flux from SGR 1806-20 with period derivative $8.3 \times 10^{-11} \rm s \, s^{-1}$confirmed this model for that repeater.