Science: Flash, bang, wallop

Was it a spaceship? A nuclear bomb? After 30 years, one deep-space mystery has been solved.

It is 1967, and the Cold War is raging. Somewhere above the former Soviet Union, an American Vela spy satellite is looking for the tell-tale signs of a nuclear test. On board is an instrument for detecting gamma- rays, electromagnetic waves in the invisible part of the spectrum that are emitted as a result of the immense shock of a nuclear explosion. Suddenly the instrument goes haywire as it records a massive flash of gamma radiation. It will be six more years before the world is told of what has happened.

The American military finally declassified the gamma-ray data from its Vela satellites in 1973, after concluding that whatever was the source of the mysterious emissions, it could not be on Earth. It was as if the satellite were in the front row of a darkened theatre, looking at the dimly lit stage of the planet. Every now and then the Vela instruments detected the popping of flashlights from somewhere in the audience - but exactly where was a mystery.

These gamma-ray flashes came from all directions in space. Their origin was not the only mystery. Scientists had no idea what could be causing these amazingly energetic outbursts of intense energy, which they calculated to be equivalent to releasing the entire energy of the Sun in just a few seconds.

Fanciful notions abounded as to what these "gamma-ray bursters" could be. Gamma-rays were, after all, integral to many a science-fiction plot. All the top space adventurers were armed with deadly gamma-ray guns, and the rays themselves were supposed to have imbued the Incredible Hulk with his unearthly powers. Suggestions that these gamma-ray bursts were messages from distant alien civilisations were quickly knocked down by the rationalists, who pointed out that gamma-rays would be an inefficient means of transmitting information, and in any case the bursts being detected seemed quite random and undecipherable. Could the bursts be the final downfall of aliens engaging in nuclear warfare, or the "exhaust" emissions of extraterrestrial spacecraft going into warp drive?

The mystery remained unresolved, although some of the greatest minds in cosmology began to think aloud about what could be causing these occasional and apparently random outbursts emanating from every corner of space. One of the problems with gamma-ray bursters was that they came and went so quickly, long before astronomers could train their instruments on the patch of space responsible. It was like standing in the centre of the Albert Hall with the lights off and trying to locate the person who had just taken a photograph with a flash.

It was not until 1997 that the theorists had an opportunity to test their ideas against the data derived from the first good observation of a gamma-ray burster. It was still not quick enough to record the precise moment of going off, but at least it witnessed the crucial moments immediately afterwards. An Italian-Dutch satellite called BeppoSAX exploited the fact that X-rays also tend to be emitted during most gamma-ray bursts. X-rays have an advantage over gamma-rays in that they are more easily pinpointed in the sky. BeppoSAX was able to do just that, by concentrating its attention on the X-ray emitted from a gamma-ray burster that "went off" on 28 February 1997. It was the first hard data on the bursters, and confirmed the theory that they were the result of some kind of immense explosive power in space.

The new data went some way towards resolving one of the main issues of contention: how far away were the bursters? One idea was that they emanated from within our own galaxy, a matter of a few hundred light years away. The other notion suggested that they were far more distant, perhaps millions or even billions of light years away. If the latter were true, it meant that the explosive forces needed to generate the gamma-rays must be truly immense, perhaps rivalling those of the Big Bang itself.

Joshua Bloom, one of the scientists who analysed the BeppoSAX data at the Institute of Astronomy in Cambridge, described the implications if the bursters came from beyond our galaxy: "If the gamma-ray bursters are at remote distances near the edge of the universe, then they are the most energetic phenomena known to humanity, releasing as much energy in a few seconds as the Sun does in 10 billion years," he said.

It was not quite certain at the time whether the location of the source could be identified exactly, but Ralph Wijers, one of the institute's leading theories on the bursters, said the data appeared to confirm a massive explosion. "The data on the afterglow of visible light agree very well with the idea that we are seeing the blast from an explosion running into the surrounding interstellar gas at nearly the speed of light."

Dr Wijers had formulated a theory to explain the gamma-ray bursters with Sir Martin Rees, the Astronomer Royal. This "fireball model" proposed that some massive event - perhaps a collision between two extremely dense neutron stars, or a huge release of energy as a result of a black hole swallowing up a neutron star - caused a gigantic fireball to bang into surrounding stellar matter and release vast quantities of gamma radiation.

Later in 1997, two other teams of astronomers, working in different regions of the electromagnetic spectrum, added further pieces to the mystery jigsaw puzzle. Greg Taylor, an astronomer at the American National Radio Astronomy Observatory in New Mexico, described how the measurement of radio waves associated with the bursters enabled the gamma-ray sources to be located with a level of accuracy unheard-of until recently. "In only one year," Dr Taylor said, "this field of research has progressed to the point that we have a position more than a million times more accurate than before."

At the same time, Mark Metzger, professor of astronomy at the California Institute of Technology, announced that a gamma-ray burster witnessed by astronomers using the Keck telescope in Hawaii showed it had the characteristic features in its spectrum showing that it must have originated in intergalactic clouds, way behind our own Milky Way. "When I finished analysing the spectrum and saw the features, I knew we had finally caught it. It was a stunning moment of revelation. Such events happen only a few times in the life of a scientist," Professor Metzger said.

The Caltech measurements, along with those from the BeppoSAX satellite, put the source of the gamma-rays at several billion light years away, more than half the distance across the universe. This made them truly awesome explosions. So awesome that some theorists had trouble explaining how they could possibly have come about, even by invoking the stupendous energy released by a massive collision between two extremely dense stars interacting with a black hole.

The latest data, released today in a joint announcement by the journals Science and Nature, may finally resolve the difficulty, and confirms the fireball shock theory proposed by Wijers and Rees. It concludes that the gamma-rays are sent out as a beam of energy, rather like a searchlight, which means that the bursters need not be as energetic as envisioned if they were sending out energy in all directions. They are still, however, the most energetic events since the Big Bang.

The latest insight came with the help of a makeshift set of four cameras, cobbled together from spare parts at the same Los Alamos Laboratory in New Mexico that was responsible for making the gamma-ray instruments on board the Vela spy satellite in the Sixties. Scientists used the cameras - called the Robotic Optical Transient Search Experiment - to observe a burster that took place on 23 January 1999.

An orbiting gamma-ray satellite fed data to the cameras, which enabled them to focus on a burster and record a series of seven images over 10 minutes, beginning 22 seconds after the satellite's initial detection of the burster. It provided unprecedented detail of a burster in action. "It's like the difference between watching two cars collide, and coming on the accident scene several hours later," said Carl Akerlof, an astronomer at the University of Michigan. "In the first case you have a much better chance of understanding what caused the crash."

Although gamma-ray bursters still retain some secrets, the 30-year mystery about what they are and where they come from seems to be finally solved.