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/2003/0814cgro_ray.html online.

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.

Los Alamos develops and applies science and technology to ensure the
safety and reliability of the U.S. nuclear deterrent; reduce the
threat of weapons of mass destruction, proliferation and terrorism;
and solve national problems in defense, energy, environment and
infrastructure.