In 1967, the Vela satellites were launched as nuclear monitors, to verify adherence to the Nuclear Test Ban Treaty. The Vela 5A and 5B satellites orbited about half way out to the Moon, where they had a view of a full terrestrial hemisphere. Their instruments were sensitive to X-rays and gamma-rays.
Almost immediately, they started detecting bursts of gamma-rays (GRBs).
did not have the double-peak characteristic of nuclear explosions.
Timing the burst arrival times between different satellites suggested that they came from all over the sky.
They appeared to be extra-terrestrial.
By 1992, over 100 hypotheses had been suggested (R. Nemiroff 1994), including
The isotropic distribution on the sky suggested that:
The Compton Gamma Ray Observatory (CGRO; 1991-2000) was designed in part to solve the mystery of the gamma ray bursts.
CGRO/BATSE detected about 1 burst per day.
The GRB distribution is isotropic. They cannot originate in the Milky Way. They must be cosmological in origin. That means they have luminosities > 1051 ergs/second (the energy the Sun puts out in its lifetime).
To determine the distance and nature of the GRBs requires finding an optical counterpart, so we can determine the morphology (galaxy, star, etc) and the distance (redshift). The BATSE positions were only accurate to about 5-10 degrees on the sky.
Beppo-SAX combined a wide-field X-ray imager with a GRB detector. About every 3 months a GRB and a simultaneous X-ray burst appeared within the view of the X-ray imager. This permitted localization to a few arc minutes, which in turn permitted optical followups.
The GRBs appear to be associated with peculiar type Ic supernovae: the collapse of a massive hydrogen-deficient Wolf-Rayet star into a black hole.
Why so bright? GRBs are not isotropic emitters. We are looking along a beam of radiation, perhaps emitted along the rotation axis of the star. Most supernovae are not GRBs because we see them at the wrong angle. The jets are ejected at relativistic velocities (V approximately c), which boosts their apparent brightness in the direction of travel.
*The Big Bang was not an explosion.