One of the longest-standing puzzles of the universe could soon be solved, writes Marcus Chown
In the 1960s, American spy satellites stumbled across something that no Pentagon strategist had ever predicted: a mysterious signal coming from the depths of space.
The satellites, in orbit to catch the Russians if they violated the test-ban treaty by exploding an H-bomb, carried special instruments that could pick up gamma rays, a type of high-energy light invisible to the human eye but generated by the fireball of a nuclear explosion.
What the satellites picked up were brief, intense bursts of gamma rays coming from space, on average once a day, always from a different place. Since the last thing the United States wanted to do was to let the Russians know it had the technical capability to detect their nuclear tests, the Pentagon kept its knowledge of the mysterious bursts of gamma rays secret. Only in 1973 did it finally declassify the information.
Somewhat prosaically, astron-omers christened the bursts of gamma rays gamma- ray bursters but their identity would elude them for a quarter of a century and become one of the longest-standing puzzles in astronomy. What makes gamma-ray bursters so enigmatic is a combination of circumstances. First, gamma-ray bursters are absurdly short-lived, lasting a few seconds, and this is simply too short a time to divine what they are (imagine trying to figure out what football is from a one-second glimpse of a game).
Since a gamma-ray burster is quite literally gone in a flash, the only thing to do is to point a big optical telescope at the place where it went off in the hope of catching the celestial object responsible red-handed. But unfortunately all instruments capable of seeing gamma rays have extremely blurry eyesight. Their view of the universe is rather like the world seen through a pane of frosted glass. The best that they can do is indicate a biggish patch of sky where the gamma-ray burst occurred. If, when astronomers looked at such a patch, they saw a single star or galaxy, then the source of gamma-ray bursters might have been discovered long ago. The trouble is there are usually hundreds or thousands of galaxies in the patch.
What has changed everything and whipped the world of astronomy into a frenzy of excitement is an obscure Dutch-Italian satellite named BeppoSAX. Orbiting high above Earth, it can pick up not only gamma rays but also X-rays. According to a theory by Britains Astronomer Royal, Sir Martin Rees, any celestial object violent enough to produce gamma-ray bursters will unavoidably generate other types of light such as X-rays.
This is exactly what BeppoSAX found in the early hours of February 28 this year. In the patch of sky where a gamma-ray burster had occurred, the instruments on board the satellite detected a fading afterglow of X- rays. It was the first time anything other than gamma rays had been found. It was a breakthrough, and it was only the beginning.
Instruments that pick up X-rays have less blurry vision than those that pick up gamma rays. The astronomers working with BeppoSAX were able to pin down the position of the gamma-ray burster more accurately than had ever been done before and pass the information rapidly to telescopes around the world. Several, including Britains William Herschel telescope, Nasas orbiting Hubble space telescope and the giant Keck telescope in Hawaii, caught a fading afterglow of ordinary visible light, which appeared to come from a faint, very distant galaxy.
Now astronomers are converging on Huntsville, Alabama, to discuss the hottest topic in astronomy. At this weeks Fourth Huntsville Gamma-Ray Burst Symposium, the latest data from BeppoSAX will become the focus of an astronomical feeding frenzy. There is a feeling that within the next year the identity of gamma-ray bursters will be unveiled.
Already there are tantalising clues. Since its launch in 1991, Nasas orbiting Compton Gamma Ray Observatory has detected more than 1 000 gamma-ray bursters and found that they occur evenly all over the sky. It seems that gamma-ray bursters may be at the very edge of the known universe.
There could scarcely be a more exciting conclusion. For if gamma-ray bursters are at the frontier of the universe, then for us to see them so brightly they must be far and away the most violent and powerful objects in creation. In the space of a few seconds, they are unleashing a burst of energy sufficient to outshine a whole galaxy of hundreds of billions of ordinary stars. Nothing, not even a supernova, in which a star explodes in an orgy of destruction, comes close to the staggering violence of a gamma-ray burster.
The theorists have their work cut out trying to guess what can liberate such destructive power. At the moment, the best candidate appears to be the collision between two black holes or two neutron stars, the super-dense relics left behind when stars die. During such an event, the tremendous gravitational energy that binds together the black holes or neutron stars is unleashed. This may drive a great fountain of trillion-degree matter outwards at almost the speed of light, natures ultimate speed limit. According to the best calculations, such collisions occur roughly once a day somewhere in the universe the rate necessary to explain gamma-ray bursters.
Although this extraordinary process appears to fit the bill, we should not get carried away. Nature has concealed her hand masterfully until now and there may still be surprises awaiting us. As the science fiction writer Arthur C Clarke is fond of repeating: The universe is not only queerer than we imagine, it is queerer than we can imagine.