Cosmic Dust Blocks Biggest Space Explosions

Some of the most gigantic explosions in space are curiously hard to see. That's because space dust is blocking the view from Earth, suggests the largest study of these blasts, so-called dark gamma-ray bursts.

Dust between these explosions and the Earth, it turns out, is the main reason that so-called dark gamma-ray bursts are so faint, but the distance of the bursts from Earth also is important, the new study suggests. These bursts shine brightly in the gamma and X-ray parts ranges of light, but show barely a spark of visible light.

Gamma-ray bursts are fleeting events that last from less than a second to several minutes, and are among the most energetic events in the universe. The afterglows from these violent outbursts last longer than the initial explosion, but have less energetic radiation.

An afterglow can last weeks, even years, after the initial explosion, and can tell scientists about the objects that become gamma-ray bursts, and about star formation in the early universe.

All gamma-ray bursts have afterglows that give off X-rays, but only about half of them were found to give off visible light, with the rest remaining mysteriously dark. The new study confirms previous work that suggested that obscuring dust was the explanation behind why they appear so dim.

Gamma-ray bursts are detected by orbiting observatories that can pick up their high-energy radiation. With data from a special gamma-ray-burst-detecting instrument and NASA's Swift satellite, astronomers determined the amount of light emitted by the afterglow of some bursts in different wavelengths, all the way from high energy X-rays to the near-infrared.

The astronomers used this information to measure the amount of obscuring dust that the light passed through en route to Earth. Previously, astronomers had to rely on rough estimates of the dust content.

A significant proportion of bursts are dimmed to about 60 to 80 percent of their original intensity by obscuring dust, but this effect is exaggerated for the very distant bursts, letting the observer see only 30 to 50 percent of the light, the researchers found.

Most dark gamma-ray bursts are therefore simply those that have had their small amount of visible light completely blocked before it reaches Earth.

NASA launched the Swift satellite at the end of 2004. From its orbit above the Earth's atmosphere it can detect gamma-ray bursts and immediately relay their positions to other observatories so that the afterglows can be studied.

In the new study, astronomers combined Swift data with new observations made using GROND ? a dedicated gamma-ray burst follow-up observation instrument, which is attached to the 7.2 feet (2.2 meter) MPG/ESO telescope in La Silla, Chile.

GROND can observe a burst within minutes of an alert coming from Swift, and it can observe simultaneously through seven filters covering both the visible and near-infrared parts of the light spectrum.

"Compared to many instruments on large telescopes, GROND is a low-cost and relatively simple instrument, yet it has been able to conclusively resolve the mystery surrounding dark gamma-ray bursts," said Jochen Greiner, study team member from the Max-Planck Institute in Germany.

The researchers report their findings in the Dec. 16 issue of the journal Astronomy & Astrophysics.

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