Light dawns on dark gamma-ray bursts
Astronomers conclude that most dark gamma-ray bursts are those that have had their small amount of visible light completely stripped away before it reaches Earth.
December 16, 2010
Gamma-ray bursts are among the most energetic events in the universe, but some appear curiously faint in visible light. The biggest study to date of these dark gamma-ray bursts, using the GROND instrument on the 2.2-meter MPG/ESO telescope at La Silla in Chile, has found that these gigantic explosions don't require exotic explanations. Their faintness is now fully explained by a combination of causes, the most important of which is the presence of dust between Earth and the explosion.
Artist's impression of a dark gamma-ray burst.
Photo by ESO
Gamma-ray bursts (GRBs), fleeting events that last from less than a second to several minutes, are detected by orbiting observatories that can pick up their high-energy radiation. Thirteen years ago, however, astronomers discovered a longer-lasting stream of less energetic radiation coming from these violent outbursts, which can last for weeks or even years after the initial explosion. Astronomers call this the burst's afterglow.
While all GRBs have afterglows that give off X-rays, only about half of them were found to give off visible light, with the rest remaining mysteriously dark. Some astronomers suspected that these dark afterglows could be examples of a whole new class of GRBs, while others thought that they might all be at great distances. Previous studies had suggested that obscuring dust between the burst and us might also explain why they were so dim.
"Studying afterglows is vital to further our understanding of the objects that become gamma-ray bursts and what they tell us about star formation in the early universe," said Jochen Greiner from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.
NASA launched the Swift satellite at the end of 2004. From its orbit above Earth's atmosphere, it can detect GRBs 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 GRB follow-up observation instrument. In doing so, astronomers have conclusively solved the puzzle of the missing optical afterglow.
What makes GROND exciting for the study of afterglows is its fast response time — it can observe a burst within minutes of an alert coming from Swift using a special system called the Rapid Response Mode — and its ability to observe simultaneously through seven filters covering both the visible and near-infrared parts of the spectrum.
By combining GROND data taken through these seven filters with Swift observations, astronomers were able to accurately determine the amount of light emitted by the afterglow at widely differing wavelengths, all the way from high-energy X-rays to the near-infrared. The astronomers used this information to directly 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.
The team used a range of data, including their own measurements from GROND, in addition to observations made by other large telescopes including the European Southern Observatory’s (ESO) Very Large Telescope (VLT), to estimate the distances to nearly all of the bursts in their sample. While they found that a significant proportion of bursts are dimmed to about 60–80 percent of the original intensity by obscuring dust, this effect is exaggerated for the very distant bursts, letting the observer see only 30–50 percent of the light. The astronomers conclude that most dark gamma-ray bursts are therefore simply those that have had their small amount of visible light completely stripped away before it reaches us.
"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 Greiner.