Gamma-ray bursts, which were first discovered by satellites originally intended to scan for nuclear weapon detonations, probably have multiple sources. They range from bursts of ten milliseconds to several hours, and also have differing energy levels.
“The near complete lack of observational constraint led to a profusion of theories, including evaporating black holes, magnetic flares on white dwarfs, accretion of matter onto neutron stars, antimatter accretion, supernovae, hypernovae, and rapid extraction of rotational energy from supermassive black holes, among others.”
Note that the highest energy bursts currently have no explanation of any kind. “No known process in the Universe can produce this much energy in such a short time.”