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Spotting a Ring of Fire
28 September 2000 7:00 pm
A fortuitous intervention by a star may have given scientists their first proof that gamma ray bursts produce fiery rings expanding into the universe. A team of astrophysicists think that they have spotted such a ring thanks to an effect called microlensing, in which a star accidentally acts as a telescope.
About once a day, a sudden explosion of gamma rays rains down on Earth from a random corner of the universe. Theorists believe the initial explosion powers an expanding spherical shock wave that crashes into the surrounding gas at nearly the speed of light. The collision lights a cosmic fire at the sphere's surface that, if you could see it, would look like a glowing ring. As the wave expands and the fire fades, the afterglow changes "color" from x-ray to optical light to radio waves. Unfortunately, the predicted ring is over a million times too small to resolve with the most powerful telescopes, so it seemed impossible to test the theory.
But last March, the gamma ray burst GRB000301C changed all that. Rapid follow-up observations with radio and optical telescopes caught an unexpected sudden brightening in the otherwise smooth fade-out. "Since gamma ray bursts are usually so well behaved, this really stood out," says radio astronomer Dale Frail of the National Radio Astronomy Observatory in Soccorro, New Mexico. At the time, Frail speculated that the shock wave brightened when it passed a lump of gas.
But there may be a different explanation, writes Peter Garnavich of the University of Notre Dame in a paper accepted for publication in the Astrophysical Journal Letters. The change in the burst's brightness appears to be exactly the same at radio and optical frequencies; this can happen, say Garnavich and his collaborators Avi Loeb and Kris Stanek from the Center for Astrophysics in Cambridge, Massachusetts, only if part of the expanding ring passed behind a star located exactly between Earth and the ring itself. The star's gravity would bend the light and magnify the ring by as much as a factor of 2, precisely as Loeb and his student Rosalba Perna had predicted in a 1998 paper. The large magnification implies that the width of the ring is between 7% and 20% of its radius, says Stanek.
"This is a more radical idea than ours, and it is a great result," says Frail. But he cautions that the data are too sparse to prove unambiguously that microlensing caused the brightening.