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ESA Science News
http://www.esa.int/science/

26 June 2003

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

Cold War intrigue, international politics and hi-tech astronomy were
the key ingredients for one of the most amazing and mysterious
scientific discoveries of all time, which took place exactly 30 years
ago. 

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. But there was an obvious lack of
data -- it was not even possible to determine whether the blasts
happened in our own Galaxy, or billions 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 

Related links

* Seeing the Universe in the gamma-ray wavelengths
   http://www.esa.int/esaSC/SEM3A2T1VED_index_0.html
* Seeing the Universe in X-ray wavelengths
   http://www.esa.int/esaSC/SEMTA2T1VED_index_0.html
* Integral -- tracking extreme radiation across the Universe
   http://www.esa.int/esaCP/ESAI0BTHN6D_Expanding_0.html
* A gamma-ray burst bonanza
   http://www.esa.int/esaSC/SEMIVX8YFDD_exploring_0.html
* Integral's first look at the gamma-ray Universe
   http://www.esa.int/esaCP/ESADW18708D_index_0.html

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