Gamma-ray detectives close in on 30-year-old mystery

Discovered in 1973, gamma-ray bursts (GRBs) are the most energetic explosions in the Universe and, even today, astronomers have still not found out what is triggering them. However, the situation is not exactly the same as it was three decades ago - ESA's space missions Integral and XMM-Newton are closing in on the answers.

In 1963, the world's superpowers signed the Nuclear Test Ban Treaty, which prevented the tests of nuclear weapons underwater, in the atmosphere or in outer space. To make sure the Treaty was not being violated, the United States launched a series of military satellites, called Vela, equipped with X-ray, gamma-ray and neutron detectors. All three types of emission are expected from a nuclear blast.

The Vela satellites did not detect any violations, but they did detect something unexpected — sixteen blasts in the gamma-ray range, but without the characteristics of a nuclear weapon. Registered between 1969 and 1972, they did not even come from the Earth's surface. This puzzled the scientists, as these 'bursts' must either be very close or very powerful. In 1973, Ray W. Klebesadel and his colleagues published the famous 'discovery paper', Observations of Gamma-Ray Bursts of Cosmic Origin, but without a convincing explanation for the events.

The mystery excited astronomers as over 200 distinct theories were soon proposed to explain the bursts. However, there was an obvious lack of data — it was not even possible to determine whether the blasts happened in our own Galaxy, or thousands of millions of light-years away.

Only when more gamma-ray satellites were launched did things begin to improve. NASA's Compton Gamma Ray Observatory was launched in 1991, to which European scientific institutes and ESA contributed with the Imaging Compton Telescope. Since then, this satellite has discovered one GRB happening randomly almost every day, coming from any direction in space and lasting for a few seconds. The uniform distribution of GRBs across the sky convinced astronomers that they most likely occur outside the Milky Way. If they were inside it, they should be seen mainly near the spiral arms, where stars are more numerous.

This idea was supported in the late 1990s when apparent counterparts to these GRBs were identified in distant galaxies. If GRBs are so far away, however, their energy release must be gigantic, with the intensity of millions and millions of Suns. Thus the core mystery is not yet solved: what kind of astronomical object could explode so catastrophically?

Thanks to missions like ESA's telescopes Integral and XMM-Newton, the solution is now closer. Integral, launched in October 2002, is today the best-prepared telescope for an in- depth study of GRBs. When a GRB occurs in the centre of Integral's field of view about once a month, four different instruments 'dissect' the event simultaneously. XMM-Newton complements Integral's work by aiming at the 'scene of the crime' as quickly as possible, to analyse the afterglow of the burst in X-ray wavelengths.

These and other observations have recently helped astronomers to select two favourite hypotheses for GRBs. They could be generated by colliding neutron stars (ultra-dense dead stars) or they could be caused by the explosions of supermassive stars at the end of their lives, so-called 'hypernovae'. Recent results provide further evidence that at least some GRBs are associated with hypernovae explosions.

Christoph Winkler, ESA's Integral Project Scientist, said, "The hypernova model sounds convincing, but others cannot still be ruled out. Of course the situation of GRB research has changed dramatically in the last decade, but we still need many more measurements to reach a final answer to what is causing these events."

Fred Jansen, ESA's XMM Project Scientist, agrees on the need for both more statistics and cooperative work and said, "We try to follow Integral announcements as quickly as we can, because we know that the solution to the mystery will come from observations with different telescopes."

With such detectives on this case, no doubt the depths of space won't be able to keep their secret for long. It's just a matter of time…