Gamma-ray bursts (GRBs) are the most luminous physical phenomena in the universe known to the field of astronomy. They consist of flashes of gamma rays that last from seconds to hours, the longer ones being followed by several days of X-ray afterglow. These flashes occur at apparently random positions in the sky about once per day.

The hypothetical objects that produce gamma-ray bursts are called gamma-ray bursters. There are two types of these bursters, long-duration bursters and short duration bursters.

There is now almost universal agreement in the astrophysics community that the long-duration bursts (> 2 sec) are associated with the beamed energy from a specific kind of hypernova event. The death of supermassive stars once its silicon "burning" is complete with a Zero Age Main Sequence (ZAMS) mass between 40 and 100 solar masses causes a direct collapse of the core to a black hole (vs. a slower "fall back" collapse if the ZAMS mass is between 25 and 40 and no black hole creation at all in stars with a ZAMS mass between 100-250 solar masses due to the resultant pair-instability supernova leaving no core remnant at all). The close connection between GRB's and Type Ib/c supernovae shows that the progenitor stars are almost exclusively low metallicity (at ZAMS), fast rotating Wolf-Rayet stars.

The angular momentum of the fast rotating Wolf-Rayet star causes the collapsing object to form a spinning accretion torus around the rapidly rotating black hole (rather like water spiraling round a plug-hole). The rotation axis area (polar regions) is quickly cleared of gas such that just 10 seconds after the "burning" ended it has around a tenth of the density of the equatorial region, allowing the energy to be released in two jets along the rotation axis. If Earth happens to lie along the rotation axis, it receives a huge burst of gamma-rays. If the Earth does not lie along the rotation axis then an x-ray flash will be observed as the relativistic jet and attendant shock interacts with the material expelled earlier in a stellar wind by the progenitor star.

The most promising model for the short duration bursts (< 2 sec but typically averaging 0.3 sec) is that developed by Martin Rees in the 1990s. In this model two neutron stars coalesce or a neutron star is devoured by a black hole. This causes an enormous release of gravitational potential energy. The lack of material around such a system means that the energy release stops as soon as the merger is complete (hence the short duration of the burst).