Originally thought to be gamma ray bursts (GRBs), soft gamma repeaters (or soft gamma ray repeaters, SGRs) were recognised as a distinct class of objects in the 1980s when it was discovered they were recurrent, and not one-off events like GRBs. They are the brightest known recurrent burster, emitting soft (low-energy) gamma rays (hence the name) and hard (high-energy) X-rays in bursts which occur at irregular intervals and typically last 0.1 seconds.
Soft gamma repeaters are now known to be magnetars, isolated neutron stars with ultra-strong magnetic fields. They emit X-rays in their quiescent state, and outbursts occur when the intense magnetic field shifts, cracking the crust of the neutron star in what is known as a starquake. The magnetic energy released ejects electrons and positrons at high speeds and creates a sudden burst of soft gamma rays and hard X-rays. This mechanism accounts for bursts typical of SGRs (i.e. fainter than GRBs).
More rarely, SGRs emit giant bursts which are thought to arise when the magnetic field becomes unstable and undergoes a large-scale rearrangement. Such a giant burst was observed in 1998 from SGR 1900+14. Originating 20,000 light years away, it was so large that detector readings on spacecraft (including those pointed away from the burst) went off-scale. It also affected radio transmissions here on Earth by ionising the upper atmosphere. All this from a neutron star located on the other side of the Galaxy!
SGRs show pulse periods clustered between 6 and 12 seconds, and observations of one SGR (SGR 1806-20) has revealed a spin-down rate of 0.26 seconds/century. This is much faster than what is observed for pulsars, and is attributed to magnetic braking by the colossal magnetic field. Such a rapid spin-down rate means that SGRs have very short lifespans (about 10,000 years), and the fact that only 5 are known supports this conclusion.
It is now thought that SGRs evolve quickly, decreasing their rotation rate and magnetic field strength, to become anomalous X-ray pulsars. The pulse periods, spin-down rates and quiescent X-ray properties of these two classes of object are similar, with the main difference being the level of activity observed.