gamma-ray burster

An astronomical source of a transient burst of gamma-radiation and X-rays. The bursts are intense and short, lasting for between a few milliseconds and a few tens of seconds. Gamma-ray bursters were first discovered by chance in the late 1960s by military satellites designed for monitoring nuclear weapons tests and have since been observed by a variety of spacecraft carrying appropriate detectors. In 1979 a single burst, which seemed to come from the Large Magellanic Cloud, was detected simultaneously by nine satellites. Monitoring by the Compton Gamma Ray Observatory (GRO) showed that bursts occur about twice a day, at random positions all over the sky. It has recorded several thousand. Though the Compton GRO was able to determine the positions of the bursters with greater accuracy than was previously possible, the positions were still not accurate enough to allow optical identification. In 1997, however, the BeppoSAX satellite, with the help of its narrow-field X-ray camera, was able to pinpoint the position of gamma-ray bursters precisely enough for them to be identified optically, and for radio emission to be detected. The first optical spectrum of a gamma-ray burster, obtained at the Keck Observatories, showed it to be at a remote cosmological distance, about halfway to the edge of the observable universe. This implies that the energy output is immense. For a few seconds the burster emits more than a million times more energy than a whole galaxy. Though many theories have been advanced, the precise mechanism is not known. Some of the more favoured theories involve the merger of two neutron stars.