RHESSI News Item

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


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

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

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

"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