gamma-ray burstsIntense flashes of hard X-rays or gamma rays, detected at energies up to one million electronvolts. They are of short duration (0.1–1000 seconds) and were discovered by US Air Force satellites in 1967 but not declassified until 1973. There are sharp temporal features in the burst time profile; this allows the measurement of differences in arrival times of wavefronts of the order of a few milliseconds over baselines separated by hundreds of light-seconds. For the strongest and most rapidly varying bursts, such measurements yield angular resolutions of the order of arc seconds. The most intense burst observed so far lies within the supernova remnant N49 in the Large Magellanic Cloud. The BATSE experiment on the Compton Gamma Ray Observatory has detected hundreds of γ-ray bursts, averaging about one per day. Measurements have revealed that the distribution of the bursts is consistent with isotropy: they are uniformly distributed across the sky. Their origin still remains a mystery. γ-ray emission lines in their spectra may be related to annihilation radiation redshifted by the strong gravitational field of a neutron star, and γ-ray absorption features to cyclotron absorption in intense magnetic fields. The rapid temporal structure, including the periodic emission, is generally assumed to point to neutron star origins for γ-ray bursts, although sources at cosmological distances cannot be ruled out. The most probable energy source is thought to be either gravitational or nuclear in origin.
gamma-ray bursts[′gam·ə ‚rā ‚bərsts]
Intense blasts of soft gamma rays of unknown origin, which range in duration from a tenth of a second to tens of seconds and occur several times a year from sources widely distributed over the sky.