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.

This is an artist's concept of RHESSI's discovery. The spacecraft is in the foreground, facing the Sun. As it observed giant explosions in the Sun's atmosphere, a distant gamma-ray burst, represented by a white sphere and ring, flared up beyond the edge of the Sun. The gamma-ray burst happened to be in RHESSI's field of view, and its light was recorded and analyzed. Credit: NASA/Walt Feimer

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.