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Lining Up an Explanation

(From the Sept/Oct 2003 issue of StarDate magazine)

Serendipitous observations of gamma-ray burst suggest magnetic field provides the juice

A chance astronomical alignment may help astronomers get a new line on gamma-ray bursts, the most powerful objects in the universe: These titanic eruptions may be driven by lines of magnetic force. The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) satellite observed a bright gamma-ray burst (GRB) in December. The craft studies solar flares, which are outbursts of X-rays, gamma rays, and charged particles from the Sun. RHESSI saw the GRB because it lined up just a few degrees from the Sun.

RHESSI’s instruments found that energy from the GRB was polarized, which means its electric and magnetic fields each “vibrate” in one direction. Such a structured alignment implies that the GRB had a strong magnetic field, said Steven Boggs, a physicist at the University of California, Berkeley, who reported the findings in the May 22 issue of Nature. In fact, Boggs said, the energy was so strongly polarized that a powerful magnetic field may have been the driving force behind the gamma-ray burst. “It appears to be a magnetically driven outflow,” Boggs said at an astronomy conference in May.

GRBs are the most powerful objects in the universe. They produce more energy than the combined output of millions of galaxies of normal stars. But most of their energy is in the form of gamma rays, which can be detected only from space. This limitation, combined with the fact that a burst typically fades in just a few seconds, makes GRBs difficult to study. Astronomers only recently determined that these bursts originate in other galaxies, far outside the Milky Way.

An emerging consensus suggests the bursts are associated with the exploding stars called supernovae. But the exact mechanism — and why some supernovae produce GRBs and others do not — remains a mystery.

RHESSI’s observations suggest that this particular GRB was driven by a powerful magnetic field twisted around by a newly formed black hole. The black hole formed when the core of a massive star collapsed. But the original star possessed a strong magnetic field, which continued to “thread” through the black hole. As the black hole spun, it twisted the magnetic field lines until the field “snapped,” blasting the outer layers of the massive star into space.

Boggs said that confirmation of RHESSI’s finding, plus observations of the magnetic fields of more gamma-ray bursts, are required before astronomers can apply this model to GRBs in general. -- Damond Benningfield