IN THE first month of the new millennium, astronomers witnessed the most
distant explosion ever seen from Earth. At a conference in Rome last week, they
predicted that mighty gamma-ray bursts, like the one on 31 January, might reveal
secrets from the far-flung corners of the early Universe.
“It’s absolutely thrilling,” says astronomer Ralph Wijers of the State
University of New York at Stony Brook, who attended the Second Workshop on
Gamma-Ray Bursts in the Afterglow Era. “We’ve been speculating for a while that
the bursts are the brightest things in the Universe, and we’ll see them as far
away as nature made them. But it’s another thing to actually have this
ܰ.”
For three decades after they were first spotted by orbiting observatories,
these bright flashes of gamma rays mystified astronomers. Typically they flash
somewhere in the sky about once a day and last from a few milliseconds to
several minutes. But the gamma-ray observatories provided no clues as to what
caused the flashes and how far away they were.
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A breakthrough came in 1997, when astronomers first snapped a gamma-ray
burst’s “afterglow”—the visible remnant that fades over several days. By
looking at the spectral fingerprints of these afterglows, astronomers have
calculated that the bursts lay very far away and were enormously bright.
Possibly so bright, in fact, that it’s as if the Sun suddenly converted its
entire mass into pure energy.
What could cause such a stellar cataclysm? Many scientists thought neutron
stars or black holes must be colliding. But now that more than 20 afterglows
have been studied, most astronomers believe the bursts come from extraordinarily
violent supernova explosions of stars 20 to 300 times as massive as the Sun.
Michael Andersen of the University of Oulu in Finland says the observations
suggest the bursts originate in regions of star formation, clouds of dust and
gas where giant stars live short lives and explode. And several groups reported
at last week’s meeting that some of these regions seemed to contain a lot of
iron, which is abundant in debris from supernovae.
But why should only some massive stars put on such a show? “Nobody knows,”
says Andersen. Spin may be the key, he says, because fast-spinning stars may
emit jets of matter along their axis as a black hole forms inside, which might
be what we see in a gamma-ray burst. “One of the main ingredients is most likely
lots of angular momentum as the star collapses to form a black hole.”
The meeting also heard that the burst on 31 January lay 11 billion light
years away. “It’s the most distant explosion ever seen in the Universe,” says
Andersen, whose team studied the burst’s afterglow. The explosion must have
taken place when the Universe was about 1.5 billion years old—roughly a
tenth of its current age.
Because these bursts are so bright, they could revolutionise our view of the
early Universe, revealing when the first giant stars lived and died, the
distribution of matter and what chemicals made up the ancient star-forming
regions. “What everybody agrees now is that gamma-ray bursts are going to be the
great tool of cosmology,” says Andersen.
The meeting highlighted the fact that new observations have thrown up nagging
puzzles—for instance, that gamma ray bursts emit light in very different
ways. “Gamma-ray bursts are more dissimilar than anything else I can think of in
astronomy, and I think it’s disturbing an increasing amount of people,” says
Andersen.
Another mystery surrounds the many “dark” bursts, which show up in gamma ray
telescopes but don’t have any detectable afterglow. It may be that the afterglow
is simply too faint for our detectors to see.
Everyone hopes that answers will come soon thanks to new satellites such as
the international High Energy Transient Explorer (HETE-2), launched earlier this
month. The first satellite devoted to spotting gamma-ray bursts, it will beam
the news to ground-based observers who can home in on the afterglow within
seconds. “It’s going to be very exciting to see what comes in the next couple of
years,” says Andersen. “We’re going to be completely soaked with data.”
