Press Release

RHESSI’s Lucky Break May Lead to Secret of Ultimate Explosions

By SpaceRef Editor
May 29, 2003
Filed under , ,
RHESSI’s Lucky Break May Lead to Secret of Ultimate Explosions
gamma rays

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

SpaceRef staff editor.