Gamma ray bursts are a big mystery. They were first discovered about 25 years ago as sudden brilliant flashes of gamma ray light coming from space by specially designed satellites used by the US Department of Defence. These Ariel satellites were supposed to look for illegal atmospheric tests of nuclear weapons, but instead found equally powerful flashes of this nuclear radiation coming from random locations in the sky.
The flashes occur about once each day, and last anywhere from a milli second to a minute or so. Astronomers used a variety of satellites to monitor the locations of these flashes, and with the advent of the NASA Compton Gamma Ray Observatory launched in the early 1990's, have now built up a map of the sky showing where over a thousand of these flashes seem to come from. They still show what earlier studies indicated, that the flashes come from just about every direction in the sky with about equal probability. This is a very important clue as to where the objects are that are producing the flashes. If they were coming from stars in our Milky Way, the flashes should have a pattern on the sky that resembles the thin band of the Milky Way that we see on a clear night. Because they are so smoothly spread out over the sky, astronomers believe they must come either from an enormous halo region surrounding our Milky Way, or from even more remote distances at billions of light years from the Earth.
Astronomers have begun to amass enough data on where in the sky the flashes are coming from that an origin in the Milky Way's halo regions can almost be ruled out. This halo would have to extend almost all the way to the Andromeda galaxy to be consistent with the smooth distribution of points. We know that the solar system is 30,000 light years from the center of any such halo, and no evidence for such an 'offset' has been seen in the flashes. In a few more years, astronomers should also be able to see the faint enhancement of flashes coming from the halo of the Andromeda galaxy, if these gamma ray flashes have anything to do with stars, or galaxy haloes.
The flashes cannot be coming from nearby galaxies out to a few hundred million light years, because these galaxies are clustered into large families, and no evidence of such clustering is seen in the flashes. This leaves an even more remote 'cosmological' distance as the only plausible possibility. But the farther away these gamma ray bursts are coming from, the more luminous they have to be to be seen 'half way across the universe'. Astronomers are now considering possibilities such as neutron stars colliding with each other in distant galaxies. We know that what ever process is at work is not the same in each case because the bursts range in character from single events spanning a few thousandths of a second, to extremely complex multiple bursts over a minute or so.
Much effort is now being spent in monitoring the sky at optical and x-ray wavelengths so that when a burst is detected by the Compton Observatory, the same region can be searched for likely candidates by optical and x-ray observations. Thus far, no unusual galaxy or star has been singled out as the source of a single one of these bursts, although one burst came very close to an old supernova remnant in the Large Magellanic Cloud some years back.