What are gamma-ray bursts?

Gamma-ray bursts are sudden, intense flashes of gamma-rays, detected mainly in the MeV gamma-ray band. When they occur, for a few seconds they completely overwhelm every other gamma-ray source in the sky, including the Sun.

GRBs were first discovered in 1967 by the Vela military satellites, although a public announcement was only made in 1973. These spacecraft carried omni-directional gamma-ray detectors, and were flown by the US Department of Defense to monitor for nuclear explosions which might violate the Nuclear Test Ban Treaty. When these mysterious gamma-ray flashes were first detected, it was determined that they did not come from the Earth's direction, and the first, quickly abandoned suspicion was that they might be the product of an advanced extraterrestrial civilization. However, it was soon realized that this was a new and extremely puzzling cosmic phenomenon. For the next 25 years, only these brief gamma-ray flashes were observed, which vanished quickly and left no traces, or so it seemed. Gamma-rays are notoriously hard to focus, so no sharp gamma-ray “images” exist to this day: the angular “error circle” within which the gamma-ray detectors can localize them is at best several degrees, which contains thousands of possible culprits. This mysterious phenomenon led to huge interest and to numerous conferences and publications, as well as to a proliferation of theories. In one famous review article at the 1975 Texas Symposium on Relativistic Astrophysics, no fewer than 100 different possible theoretical models of GRBs were listed, most of which could not be ruled out by the observations then available.