The cosmic neighbours from hell

They are the most energetic events in the Universe - the biggest bangs since the Big Bang - yet next to nothing is known about these mysterious invisible explosions in deep space. They erupt at least once a day, releasing in a few seconds a hundred billion times the energy emitted by the Sun over an entire year. But the mystery of gamma-ray bursts could soon be solved with the planned launch this month of the first satellite dedicated to their study.

They are the most energetic events in the Universe - the biggest bangs since the Big Bang - yet next to nothing is known about these mysterious invisible explosions in deep space. They erupt at least once a day, releasing in a few seconds a hundred billion times the energy emitted by the Sun over an entire year. But the mystery of gamma-ray bursts could soon be solved with the planned launch this month of the first satellite dedicated to their study.

Scientists have known about gamma-ray bursts since 1973 when the American military declassified data it had gathered in 1967 from a Vela spy satellite. It was the height of the Cold War when the Vela was orbiting above the Soviet Union looking for the tell-tale signs of a nuclear weapons test. All of sudden its instruments were blinded by an intense flash of gamma rays, electromagnetic waves in the invisible part of the spectrum that are given off due to the intense shock of a nuclear blast.

But these gamma rays did not come from a nuclear-weapons test. It took some time for military scientists to realise that the radiation did not even come from Earth. They originated from somewhere in deep space. What caused them was a complete mystery.

The difficulty was that although these intense explosions are truly stupendous, they are incredibly fleeting, sometimes lasting no more than a couple of seconds. Attempting to locate their source was like facing the dimly lit stage of a darkened theatre and trying to guess the position of someone in the audience behind you who took 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 burst. 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 pin-pointed in the sky. BeppoSAX was able to do just that, by concentrating its attention on the X-ray emitted from a gamma-ray burst that exploded on 28 February 1997. It was the first hard data on the phenomenon, and confirmed the theory that they were the result of some kind of immense explosive power in space.

When astronomers began to calculate the energies released during a gamma-ray burst, their interest turned to astonishment. They found that in the space of a few seconds the explosive energy emitted largely in the form of gamma radiation is equivalent to the energy released by the Sun during its entire lifetime of some 10 billion years. Whatever is causing these bursts, it's a good job that they are billions of light-years away - too far to seriously damage life on Earth.

The £138m satellite due to be launched this month is designed to solve the riddle of what causes a gamma-ray burst. Named after the bird that catches its food in flight, the Swift space laboratory is a joint project between the US National Aeronautics and Space Administration (Nasa), Britain and Italy. It has three instruments on board. One is for detecting the rough location of a gamma-ray burst within seconds of it going off, and two more that can be quickly swivelled in that direction to study the burst's "afterglow" of radiation, which includes radiation in the form of X-rays, ultraviolet and visible light.

Swift is also designed to transmit information within seconds to ground-based telescopes on Earth which can also be corralled in the hunt for a gamma-ray afterglow, says Professor Ian Halliday, chief executive of the Particle Physics and Astronomy Research Council, which has contributed £3.81m to the cost of the project. "Swift is unique because it brings together space scientists and ground-based astronomers in a way that has never been achieved before - working together, they are poised to reveal the origins and complexity of the most violent phenomena in the Universe," Professor Halliday says.

Professor Keith Mason, of the Mullard Space Science Laboratory at University College London, and Britain's lead investigator on Swift's optical-UV telescope, explains the satellite significance. "Swift is the first spacecraft specifically designed to study the afterglow of gamma-ray bursts across the electromagnetic spectrum, thereby getting the big picture," he says. "Until now it has proved very difficult to observe several different wavelengths at once. For an orbiting observatory, the Earth gets in the way at different times for each spacecraft, while on ground-based telescopes you have the added complication of cloud and bad weather."

Nasa describes gamma-ray bursts as "the most puzzling and intriguing astronomical phenomena found in modern times", yet there are several theories as to what is behind them. What measurements that have been made suggest that there may be two types of bursts, one of exceptionally short duration of no more than a fraction of a second, and another version that lingers for maybe a few seconds. In both cases, scientists suspect that the burst marks the birth of a black hole - a highly dense region of such intense gravity that not even light can escape. "We believe that gamma-ray bursts may mark the birth of a black hole. Swift is designed to answer such questions," Professor Mason says.

There is mounting evidence that some gamma-ray bursts arise from the explosion of massive stars. Some theorists also suggest the bursts may emanate from spectacular mergers or collisions in space, perhaps between two neutron stars or two black holes. As Nasa explains: "The powerful blast wave ploughs the interstellar medium, with some of its debris accelerated to near light speed. This blast wave heats the surrounding gas to ultra-high temperatures and, perhaps, triggers new star formation. It is the blast wave's interaction with itself and the surrounding medium, in fact, that creates the gamma rays that we ultimately see."

Of course we can only "see" gamma rays with specialist instruments. If we could really see them with our own eyes then the bursts would look like flashbulbs going off in the sky. Swift, when it becomes operational next March, is expected to detect at least one and perhaps two a day. Scientists hope it will collect good data on at least 200 bursts over its two-year working lifetime.

At the end of that period, Swift could be shedding a lot of light on other aspects of cosmology because it will effectively be an instrument for looking back in time to when stars and galaxies arose after the Big Bang.

Gamma-ray bursts are so powerful that they can be detected from Earth even when they occur on the other side of the Universe. Because such vast distances are also a measure of time - it takes light time to reach Earth - it means that their radiation effectively allows scientists to look back to events that took place close to the Big Bang, when the Universe was created about 13 billion years ago. Gamma-ray bursts are so powerful, in fact, that scientists estimate that some of the most distant ones might have gone off when the Universe was only 5 per cent of the age it is today.

Their immense violence begs the question of what would happen if one exploded within our own galaxy. Professor Mason suggests that gamma-ray bursts in our own Milky Way galaxy may have indeed been responsible for releasing so much harmful radiation that it could have been responsible for some of the mass extinctions of life on Earth. "Even if a gamma-ray burst went off in the centre of our galaxy, which is 30,000 light-years away, it would still rival the Sun in terms of brightness," he says. "There is no reason for saying that gamma-ray bursts would not go off in our galaxy, and if one were to go off nearby it would cause serious damage."

Let's hope that the most violent events in the Universe continue to take place in very deep space well beyond our cosmic neighbourhood.