The hunt for cosmic blasts' secret

One of the most mysterious and explosive phenomena in the Universe could finally yield its secrets when Nasa launches its space mission Swift.

Gamma-ray bursts can release as much energy in a few minutes as the Sun emits in its 10-billion-year lifetime.

Yet what causes these mighty blasts is still unknown, UK scientists explained at a news conference in London on Monday.

The $250m probe, whose launch in Florida was delayed by hurricanes, could lift off on 11 November at the earliest.

"The source of gamma-ray bursts is a long-standing detective story in modern astronomy," said Professor Keith Mason, of the Mullard Space Science Laboratory (MSSL) in London and lead investigator for the Ultraviolet-Optical Telescope on Swift.

Professor Martin Ward, chair of the science committee for the Particle Physics and Astronomy Research Council (Pparc), commented: "They are the biggest bangs since the big one."

Black-hole sun

Scientists think that gamma-ray bursts are generated in two principal scenarios. In one scenario, a star collapses in on itself, giving birth to a black hole.

As stars reach the end of their life, the nuclear reactions in their cores stop. This removes the radiation pressure that keeps the star inflated, so that the core of the star collapses inwards and forms a black hole.

Material is sucked in towards the black hole and begins to circle it at high velocity. This causes a powerful jet to burst from the torrent, carrying material moving at close to the speed of light. It is this jet that carries the radiation detected on Earth as gamma-ray bursts.

"This jet contains huge amounts of energy, much of which is converted to electromagnetic radiation; gamma rays, X-rays or optical light," said Prof Mason.



Gamma-ray bursts are bright, but they are short-lived

"Because it is moving at close to the speed of light, the radiation is emitted primarily in the direction of motion of that jet. This causes the emission to be tightly focused in a particular direction, like a flashlight."

Scientists also think the bursts can form when two neutron stars collide. However, they are keeping their minds open to the possibility that other, hitherto unimagined, phenomena might cause the giant blasts.

By studying these bursts, scientists hope to peer back in time at the first stars and discover when they formed. Because the flashes of radiation are so bright, they can be seen from long distances away.

And of course, to look at the distant Universe is also to look back in time. "We can see gamma-ray bursts from collapsing stars when the Universe was 5% of its present age," said Prof Mason.

He speculated that gamma-ray bursts could even be implicated in some extinctions in Earth history: "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."

Currently, astronomers see about one or two gamma-ray bursts each month. But it is hoped that after the launch of Swift, this could go up to about 100 bursts observed per year.

Waiting game

Swift will orbit Earth in wait for a gamma-ray burst. An instrument called the Burst Alert Telescope (BAT) aboard the probe is designed to detect the flashes from these explosions.

Within about 20 seconds of a detection, Swift relays the position of the burst to the ground. As it relays this information, the spacecraft veers around to train its two onboard telescopes on the source, enabling precision measurements to be made.

The spacecraft gets its name from the speed with which it has to move in order to observe the short-lived phenomena.

"We are aiming to catch gamma-ray bursts on the hop," said Professor Alan Wells of the University of Leicester, who is lead investigator on Swift's X-ray telescope.

Gamma-ray bursts were first observed during the Cold War, when Western researchers thought that they might be the product of Soviet nuclear tests on the Moon or on other planets.

Swift will join four other satellites connected to a largely automated system that relays alerts on gamma-ray bursts in real time to scientists worldwide. This network will distribute Swift alerts via e-mail to scientists and to robotic telescopes.



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