EMBARGOED
FOR RELEASE UNTIL WED., OCT. 18, 2000, TO COINCIDE WITH
PRESS BRIEFING AT INTERNATIONAL GAMMA RAY CONFERENCE IN
ROME, ITALY
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NASA's
Ulysses spacecraft and its orbit around the sun |
Berkeley
- The afterglow of a gamma-ray burst in the southern constellation
of Carina - more distant than any high-energy flare ever
observed - has been detected by a network of spacecraft
spread over the solar system, and has been traced back to
its original explosion about 11 billion years ago.
The observations,
made by a cluster of interplanetary probes called the Interplanetary
Network, revealed that the burst probably came from a gigantic
dying star more than 30 times as massive as the sun, when
the universe was about one-tenth of its present age, said
Kevin Hurley, a physicist at the Space Sciences Laboratory
of the University of California, Berkeley, and principal
investigator of the gamma-ray burst experiment on board
NASA's Ulysses spacecraft. Ulysses is one of several spacecraft
studying the sun, and the only one that passes over the
sun's poles.
"Detection
of gamma-ray burst GRB 000131 at an extremely high red shift
of 4.5 corresponds to a distance of about 11 billion light
years away," said Hurley, whose results will be reported
today at the Science of Gamma-Ray Bursts conference in Rome,
an international colloquium on gamma ray bursts. "The light
from this gigantic flash had traveled 11 billion years before
reaching the Earth, and suggests that these explosive objects
may provide us with the longest yardsticks yet for detecting
and studying galaxies in the early universe."
Hurley's
colleagues also will report details of the gamma ray observations
at a press briefing today in Rome. Results of the observations
will appear in a December issue of the European science
journal Astronomy and Astrophysics.
All objects
in the universe are flying away from each other as a result
of the Big Bang. More distant galaxies recede faster from
Earth than nearby galaxies. This is observed as a Doppler
effect, best explained by the pitch of a train whistle,
which rises as the train approaches and diminishes as the
train speeds away. Similarly, if a galaxy is approaching
the Earth, all of the wavelengths in the galaxy's spectrum
are shifted toward the blue end of the spectrum. If the
galaxy is receding from Earth, all wavelengths in the spectrum
are shifted toward the red end in what is termed a "red
shift." Observations of GRB 000131 revealed that it is receding
and its light is shifted toward the red end of the spectrum
by a factor of 4.5, which is a significant shift, Hurley
said.
Before
these observations were recorded, the most distant gamma-ray
burst to be detected was GRB 971214, estimated to be less
than 9 billion light years away in the constellation Ursa
Major. A violent burst of gamma-ray radiation from this
object was recorded on December 14, 1997, by Italian and
U.S. satellites, including Ulysses.
"If this
were the Olympics, we'd have the gold medal now," said Hurley,
who runs the network of spacecraft, which includes the joint
European Space Agency/NASA Ulysses spacecraft, NASA's Near
Earth Asteroid Rendezvous (NEAR-Shoemaker) and Wind spacecraft,
and the Italian Bepposax spacecraft. "Still, the fact that
such a faint, distant source can produce a burst of gamma
rays which appears to be of more or less average intensity
from Earth hasn't been explained yet."
Gamma-ray
bursts are mysterious flashes of high-energy light occurring
typically about once a day somewhere in the sky. However,
their origins remain a mystery to astronomers, most of whom
believe they are enormous explosions that occur far across
the universe. Only recently has enough evidence been accumulated
to link the longest of these bursts to hypernovas: giant,
extraordinarily intense, unusual supernovas, which are dying
stars that collapse under the weight of their own mass.
At their
peak, these bursts are by far the brightest emissions of
gamma-ray radiation in the sky, Hurley said. In fact, gamma-ray
bursts are the most powerful explosions in the known universe.
They generate more energy in a few seconds than the sun
will generate in its entire lifetime.
Bursts
are detected approximately once a week by these surveillance
craft, although the Italian Bepposax spacecraft did not
detect this burst. All four spacecraft are equipped with
detectors to measure gamma rays, an energetic form of x-radiation,
Hurley said. The gamma rays are impossible to detect from
Earth because they are absorbed by the atmosphere. By timing
the arrival of gamma rays as they reach the spacecraft,
their arrival directions can be precisely determined.
"Although
this distant burst was observed on January 31, 2000, it
has taken scientists eight months to study its source,"
Hurley said. "The flash of high-energy gamma radiation appeared
at first to be relatively normal. It was neither very faint,
which might indicate that it had traveled a long way from
its source, nor very bright, which would hint at a somewhat
closer origin."
However,
when a team of Finnish and Danish astronomers observed the
object with the European Southern Observatory's Very Large
Telescope in Chile, they found a very dim optical afterglow,
suggesting that it was indeed situated at an extremely large
distance. To confirm this suspicion, they had to observe
it repeatedly, adding the results of each observation to
previous observations until the picture became clear.
Hurley
and his colleagues believe a sharply focused beam of gamma-ray
radiation probably made the observations possible. "It seems
likely that the gamma rays were emitted in a narrow beam,
like a searchlight, and we just happened to be in that line
of sight," Hurley said. "The faintness of the optical radiation,
which was not beamed, was indicative of its large distance."
Since
gamma radiation is energetic enough to travel unimpeded
throughout the entire universe, astronomers are excited
about the prospects of using gamma-ray bursts and related
optical observations to reveal the details of very distant
galaxies and matter in the universe.
"Optical
light traveling through extragalactic space on its way toward
Earth changes in various ways," Hurley said. "Atoms of hydrogen
can absorb part of it along the way and galaxies can actually
bend the light. The effect that matter and galaxies in the
very distant universe have on this distant light is one
of the factors that makes the study of gamma-ray bursts
so interesting."
Some
theories suggest that gamma-ray bursts might be detectable
out to red shifts of 10 or more - about two times farther
out than the Jan. 31 gamma-ray burst - and, thus, act as
a window on the very early universe.
"The
observation of this burst can be viewed as another rung
in the distance ladder," Hurley said. "Everything about
gamma-ray bursts has proven to be a complete surprise to
us, and this observation is a good example. Our data may
indicate that we can indeed see much farther still."
The international
Science of Gamma-Ray Bursts conference is being hosted by
Italy's Consiglio Nazionale di Ricerche, the Italian national
research organization that sponsors almost all scientific
research in Italy.
Sponsors
include the Istituto di Astrofisica Spaziale (Space Astrophysics
Institute) in Rome; the Istituto di Technolgie e Studie
delle Radiazioni Extraterrestri (Institute for Technology
and the Study of Extraterrestrial Radiation) in Bologna;
and the European Southern Observatory, a consortium of eight
European countries that operates the world's most advanced
multi-telescope observatory in Chile.
###
Images
and the latest results from the Ulysses mission and other
scientific probes studying the sun are available at:
http://ssl.berkeley.edu/ipn3/index.html
http://ulysses.jpl.nasa.gov/
http://sci.esa.int/home/ulysses/index.cfm
http://www.eso.org/outreach/press-rel/pr-2000/pr-20-00.html
