The gamma-ray bursts are intense, short flashes of gamma-rays arriving from any direction in the sky, at unpredictable times. They usually emit most of their energy in the hard-X/gamma-ray range, but they have been detected from few keV up to tens of GeV.
They were discovered in 1969 by gamma-ray detectors on-board the military satellites Vela 5A, 5B, 6A, 6B, employed by the USA to monitor against possible Russian and Chinese nuclear tests in the atmosphere, in violation of the 1963 Nuclear Test Ban Treaty. After their discovery they were also found in older data and the first event came out to be recorded on 2 July 1967. Being a result of military investigations they were classified and were announced to the scientific community only in 1973 ([Klebesadel et al. 1973]).
After their discovery several gamma-ray experiments were included
in the scientific payload of subsequent satellites, and continued
to detect GRBs. Today the instrumental sensitivity has reached about
10-8 erg cm-2 s-1 and they are then detected at a gross rate of
one per day, corresponding to a rate of 
3/day from the whole sky, taking into account the coverage.. A crucial contribution to the study of GRBs has been
gained by the launch in 1991 of the Burst And Transient Source
Experiment (BATSE [Fishman et al. 1994]) on-board the
Compton Gamma-Ray Observatory.