A soft gamma repeater is an astronomical object that sends out large bursts of gamma rays and X-rays at irregular intervals. These are now known to be magnetars.

On March 5, 1979 a powerful gamma ray burst was noted. Because a number of receivers at different locations in the solar system saw the burst at slightly different times, its direction could be determined, and it was shown to originate from a supernova remnant in the Large Magellanic Cloud

Through time, it became clear that this was not a normal gamma ray burst. The photons were less energetic (in the soft gamma ray and hard X-ray range), and repeated bursts came from the same region.

The Greek astronomer Chryssa Kouveliotou decided to test the theory that soft gamma repeaters were magnetars. According to the theory, the bursts would cause the object to slow down its rotation. In 1998, she made careful comparisons the periodicity of soft gamma repeater SGF 1806-20. The period had increased by 0.008 seconds since 1993, and she calculated that this would be explained by a magnetar with a magnetic field strength of gauss. This was enough to convince the astronomic community that soft gamma repeaters are indeed magnetars.

An unusually spectacular soft gamma repeater burst was the burst of SGR 1900+14 on August 27, 1998. Despite the large distance to the SGR (estimated at 20,000 light years), the burst had large effects on the Earth's atmosphere. The atoms in the ionosphere, which are usually ionized by the Sun's radiation by day, and recombine to neutral atoms by night, were ionized at nighttime at levels not much lower than the normal daytime level. The Rossi X-Ray Timing Explorer (RXTE), an X-ray satellite received its strongest signal to that date, even though it was directed at a different part of the sky, and should normally have been shielded from the radiation.

Four Soft Gamma Repeaters are currently (2000) known:

  • SGR 0525-66 (discovered 1979)
  • SGR 1806-20 (discovered 1979/1986)
  • SGR 1900+14 (discovered 1979/1986)
  • SGR 1627-41 (discovered 1998)

The numbers give the position in the sky - SGR 0525-66 for example has a right ascension of 5h25m and a declination of -66°. The 1979/1986 years are given that way because these have been first seen in 1979, but soft gamma repeaters were only recognized as a separate class of objects (rather than 'normal' gamma ray bursts) in 1986.