Cosmic afterglow steals the limelight

A chance cosmic alignment may have led astronomers to a clearer picture of gamma-ray bursts, flashes of high-energy radiation that rank as the most powerful explosions known in the universe.

The amplification of the afterglow of a gamma-ray burst (data points; uncertainty shown as bars) fits the curve predicted for the effect of a gravitational lens. Loeb, et al.

Gamma-ray bursts erupt at random, and spacecraft detect them about once a day. Theorists have proposed that the bursts power a spherical shock wave that slams into surrounding space at nearly the speed of light. The collision generates an afterglow that telescopes can record for days to weeks–first in X rays, then visible light, and finally radio waves. But these afterglows have been too small for a telescope to discern their spatial structure.

Now, researchers suggest they have resolved such a structure for the first time–even though its parent gamma-ray burst erupted halfway across the universe. Thanks to some gravitational sleight of hand, researchers last March were treated to a magnified view of a gamma-ray burst dubbed GRB 00301C. They report their findings in an upcoming Astrophysical Journal Letters.

According to general relativity, a massive object that lies between a more distant body and Earth can act as a gravitational lens, bending and magnifying light from the distant body. The visible-light afterglow from GRB 000301C appears to have passed through such a lens: Instead of rapidly fading, the light remained bright. It peaked almost 4 days after spacecraft detected the parent burst.

Radio and visible-light wavelengths were amplified by about the same amount–another hallmark of a gravitational lens, note coauthors Peter M. Garnavich of the University of Notre Dame in Indiana and Abraham Loeb and Kris Z. Stanek of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

The duration and amount of brightening match the pattern expected when a lens amplifies the light from a fast-moving shock wave. Loeb and Rosalba Perna, then at Harvard-Smithsonian, predicted 2 years ago that the afterglow would appear as a thin, rapidly expanding ring. As the ring grows larger and sweeps behind the lens, magnification would reach a maximum.

Garnavich’s team calculates that the lens weighs about half as much as the sun and may lie in a galaxy about halfway between the burst and Earth.

Andrew S. Fruchter of the Space Telescope Science Institute in Baltimore says the idea of the lens is “fascinating, and they might be right.” Proof will have to await studies of many more bursts, he says.

Loeb estimates that less than 1 percent of all afterglows are magnified by a lens. As more examples are found, researchers will be better able to gauge both the wallop packed by a burst and the nature of the medium encountered by the shock wave, he says. On Oct. 9, NASA launched the High-Energy Transient Explorer, which promises to record many more bursts and rapidly alert ground-based telescopes to look for afterglows.

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