The Soft Gamma-Ray Repeaters

Abstract

The Soft Gamma-Ray Repeaters (SGR) are sources of brief intense outbursts of low energy gamma rays. Most likely they are a new manifestation of neutron stars. Three sources were known prior to the launch of CGRO, and bursts from two of these have been seen by BATSE. However, no new sources have been discovered, which means that they are either a very rare type of object, or the bursting stage is a very short phase of the life of many neutron stars. The BATSE and RXTE observations have shown that SGR 1806−20 remains the most prolific of the three. An important breakthrough was that a BATSE burst was also seen by ASCA, allowing a good location to be determined. A quiescent X-ray source was found at this location, as well as a compact and an extended radio source. These suggested that this is a pulsar-powered supernova remnant. However, also present at this location was a heavily reddened star that is consistent with being a rare Luminous Blue Variable. In addition, there appear to be compact and extended dust sources present. The relationship of these different components to the emitter of the gamma-ray bursts remains unclear. BATSE also detected bursts from SGR 1900+14. Using the network synthesis technique, relatively small error boxes were determined, and a candidate ROSAT X-ray counterpart was found. A search for a radio counterpart there did not reveal any sources. However, a pair of heavily reddened stars that appear to be variable M5 supergiants were consistent with this location: their spectra are remarkably similar. As for SGR 1806−20, compact and extended dust sources were also discovered. But again, it is not currently understood what, if any, relevance these objects have to the source of the gamma-ray bursts. To date, BATSE has not seen any bursts from SGR 0525−66. The error box for this source is consistent with the N49 supernova remnant in the LMC, and a possible soft X-ray counterpart has been found. However, in spite of detailed searches, no point-like counterpart has been detected at any other wavelength. Given the bizarre counterparts to the other two SGR, this lack has only increased the confusion regarding these enigmatic sources. Assuming the sources are correctly identified as being in our Galaxy and in the LMC, it is not easy to model the super-Eddington luminosities during the bursts. The most popular models currently span a very large range from ultrastrong magnetic field scenarios (“magnetars”) to self-absorbed synchrotron emission from cooling relativistic electrons in a moderate neutron star magnetic field to pulsar glitches to accretion of planets or comets onto neutron stars.