Biggest cosmic explosions also may propel fastest objects in universe(Forwarded)

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03-106

Biggest cosmic explosions also may propel fastest objects in universe

LOS ALAMOS, N.M., Aug. 13, 2003 -- The most powerful explosions in the universe,
gamma-ray bursts, may generate the most energetic particles in the universe,
known as ultrahigh-energy cosmic rays (UHECRs), according to a new analysis of
observations from NASA's Compton Gamma-Ray Observatory.

Los Alamos National Laboratory researchers and their colleagues from the U.S.
Naval Research Laboratory in Washington and the University of Alabama at
Huntsville report in the Aug. 14 edition of Nature of a newly identified pattern
in the light from these enigmatic bursts that could be explained by protons
moving within a hair's breadth of light speed.

These protons, like shrapnel from an explosion, could be UHECRs. Such cosmic
rays are rare and constitute an enduring mystery in astrophysics, seemingly
defying physical explanation, for they are simply far too energetic to have been
generated by well-known mechanisms such as supernova explosions.

"Cosmic rays 'forget' where they come from because, unlike light, they are
whipped about in space by magnetic fields," said lead author Maria Magdalena
Gonzalez of Los Alamos' Neutron Science and Technology Group and a graduate
student at the University of Wisconsin. "This result is an exciting chance to
possibly see evidence of them being produced at their source."

Gamma-ray bursts -- a mystery scientists are finally beginning to unravel -- can
shine as brilliantly as a million trillion suns, and many may be from an
unusually powerful type of exploding star. The bursts are common yet random and
fleeting, lasting only seconds.

Cosmic rays are atomic particles (for example, electrons, protons or neutrinos)
moving close to light speed. Lower-energy cosmic rays bombard the Earth
constantly, propelled by solar flares and typical star explosions. UHECRs are a
hundred-million times more energetic than the particles produced in the largest
human-made particle accelerators.

Scientists say the UHECRs must be generated relatively close to the Earth, for
any particle traveling farther than 100 million light years would lose some of
its energy by the time it reached us. Yet no local source of ordinary cosmic
rays seems powerful enough to generate a UHECR.

The Gonzalez-led paper focuses not specifically on UHECR production but rather a
new pattern of light seen in a gamma-ray burst. Digging deep into the Compton
Observatory archives (the mission ended in 2000), the group found that a
gamma-ray burst from 1994, named GRB941017, appears different from the other
2,700-some bursts recorded by the Compton Observatory. This burst was located in
the direction of the constellation Sagitta, the Arrow, likely 10 billion light
years away.

What scientists call gamma rays are photons (light particles) covering a wide
range of energies, in fact, more than a million times wider than the energies
our eyes register as the colors in a rainbow. Gonzalez's group looked at the
higher-energy gamma-ray photons. The scientists found that these types of
photons dominated the burst: They were at least three times more powerful on
average than the lower-energy component yet, surprisingly, thousands of times
more powerful after about 100 seconds.

That is, while the flow of lower-energy photons hitting the satellite's
detectors began to ease, the flow of higher-energy photons remained steady. The
finding is inconsistent with the popular "synchrotron shock model" describing
most bursts. So what could explain this enrichment of higher-energy photons?

"One explanation is that ultrahigh-energy cosmic rays are responsible, but
exactly how they create the gamma rays with the energy patterns we saw needs a
lot of calculating," said Brenda Dingus of Los Alamos, a co-author on the paper.
"We'll be keeping some theorists busy trying to figure this out."

A delayed injection of ultrahigh-energy electrons provides another way to
explain the unexpectedly large high-energy gamma-ray flow observed in GRB
941017. But this explanation would require a revision of the standard burst
model, said co-author Charles Dermer, a theoretical astrophysicist at the U.S.
Naval Research Laboratory in Washington.

"In either case, this result reveals a new process occurring in gamma-ray
bursts," Dermer said.

Gamma-ray bursts have not been detected originating within 100 million light
years from Earth, but through the eons these types of explosions may have
occurred locally. If so, Dingus said, the mechanism her group saw in GRB941017
could have been duplicated close to home, close enough to supply the UHECRs we
see today.

Other bursts in the Compton Observatory archive may have exhibited a similar
pattern, but the data are not conclusive. NASA's Gamma-ray Large Area Space
Telescope, scheduled for launch in 2006, will have detectors powerful enough to
resolve higher-energy gamma-ray photons and solve this mystery.

The Compton Gamma Ray Observatory was the second of NASA's Great Observatories
and the gamma-ray equivalent to the Hubble Space Telescope and the Chandra X-ray
Observatory. Compton was launched aboard the Space Shuttle Atlantis in April
1991, and at 17 tons, was the largest astrophysical payload ever flown at that
time. At the end of its pioneering mission, Compton was deorbited and re-entered
the Earth's atmosphere on June 4, 2000.

Co-authors of the Nature report also include Ph.D. graduate student Yuki Kaneko,
Dr. Robert Preece, and Dr. Michael Briggs of the University of Alabama in
Huntsville. The research was funded by NASA and the Office of Naval Research.
Los Alamos work was funded through the Laboratory Directed Research and
Development program.

More information is available from the NASA World Wide Web site at
http://www.gsfc.nasa.gov/topstory/20...4cgro_ray.html

Los Alamos National Laboratory is operated by the University of California for
the National Nuclear Security Administration (NNSA) of the U.S. Department of
Energy and works in partnership with NNSA's Sandia and Lawrence Livermore
national laboratories to support NNSA in its mission.

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