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Donald Savage
Headquarters, Washington           May 28, 2003 
(Phone: 202/358-1547)

Bill Steigerwald
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-5907)

Robert Sanders
University of California, Berkeley, Calif.
(Phone: 510/643-6998)

RELEASE: 03-180

RHESSI'S LUCKY BREAK MAY LEAD TO SECRET OF ULTIMATE 
EXPLOSIONS

     A chance observation by a NASA satellite, designed to 
study the sun, may have uncovered one of the most important 
clues yet obtained about the mechanism for producing gamma 
ray bursts, the most powerful explosions in the universe. 

The Reuven Ramaty High-Energy Solar Spectroscopic Imager 
(RHESSI) satellite was snapping pictures of solar flares on 
December 6, 2002. Unexpectedly, RHESSI caught an extremely 
bright gamma ray burst in the background, over the edge of 
the sun. The image revealed, for the first time, gamma rays 
in such a burst are polarized. The result indicates intense 
magnetic fields may be the driving force behind these awesome 
explosions.

Solar flares are tremendous explosions, in the atmosphere of 
the sun, powered by the sudden release of magnetic energy. 
Gamma ray bursts are remote flashes of gamma ray light that 
randomly pop off, about once a day, briefly shining as bright 
as a million, trillion suns. Recent observations suggest 
bursts may be produced by a special kind of exploding star 
(supernova). But not all supernovae generate gamma ray 
bursts, so the physics of how a supernova explosion can 
produce a burst of gamma rays is unclear.

Two University of California, Berkeley (UCB), researchers, 
Dr. Steven Boggs, assistant professor of physics, and Dr. 
Wayne Coburn, a postdoctoral fellow at the UCB Space Sciences 
Laboratory, are presenting their findings today during a 
press conference at the American Astronomical Society meeting 
in Nashville, Tenn. Their paper about this discovery was 
published in the May 22 issue of Nature.

"RHESSI was sent into space to uncover the secrets of solar 
flares, the largest explosions in our Solar System, so I am 
delighted that it has been able to serendipitously provide 
new information about gamma ray bursts, the largest 
explosions in the whole universe," said Dr. Brian Dennis, 
RHESSI Mission Scientist at NASA's Goddard Space Flight 
Center, Greenbelt, Md.  "Curiously, magnetic fields seem to 
be driving both the local solar flares and the distant gamma 
ray bursts, two immensely powerful events," Dennis added.

The strong polarization measured by RHESSI provides a unique 
window on how these bursts are powered, according to Boggs. 
He interprets the measurements to mean the burst originates 
from a region of highly structured magnetic fields, stronger 
than the fields at the surface of a neutron star, until now, 
the strongest magnetic fields observed in the universe. "The 
polarization is telling us the magnetic fields themselves are 
acting as the dynamite, driving the explosive fireball we see 
as a gamma ray burst," he said.

The gamma rays measured by RHESSI were about 80 percent 
polarized, consistent with the maximum possible polarization 
from electrons spiraling around magnetic field lines. The 
spiraling causes electrons to produce light by "synchrotron 
radiation." Polarized light, familiar to most of us as the 
reflected light blocked by Polaroid sunglasses, is light with 
its magnetic and electric fields primarily vibrating in one 
direction, not randomly. Such coherence implies an underlying 
physical symmetry, in this case, aligned magnetic fields.

Though the electrons are probably accelerated to nearly the 
speed of light in shock waves, the fact the gamma rays are 
maximally polarized implies the shock waves themselves are 
driven by an underlying strong magnetic field.

"The amount of polarization they found is so intense, that it 
looks like it's pure synchrotron radiation and nothing else. 
All the other theories are going to have to bite the dust 
now," said Dr. Kevin Hurley, a UCB gamma ray burst physicist. 
Since 1990, Hurley has operated the Third Interplanetary 
Network (IPN3) of six satellites linked together to pinpoint 
gamma ray bursts and immediately alert astronomers. However, 
for such a novel measurement, further independent 
confirmation is crucial, Boggs added.

The discovery of polarization reveals how a gamma ray burst 
is powered, through the generation of a strong, large-scale 
magnetic field. The next question is: Why do some supernovae 
lead to a strong, organized magnetic field? This might be a 
question we can only address through theory, but the pieces 
of evidence are in place for theorists to unravel, Boggs 
concluded.

For more information about the research on the Internet, 
visit:

http://www.gsfc.nasa.gov/topstory/2003/0528rhessigrb.html

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