Headlines > News > Light dawns on dark gamma-ray bursts

Light dawns on dark gamma-ray bursts

Published by Klaus Schmidt on Fri Dec 17, 2010 9:18 am via: Eurekalert
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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 gamma-ray bursts [1] 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 gamma-ray bursts, while others thought that they might all be at very great distances. Previous studies had suggested that obscuring dust between the burst and us might also explain why they were so dim.

This artist’s impression shows a dark gamma-ray burst in a star forming region. Gamma-ray bursts are among the most energetic events in the Universe, but some appear curiously faint in visible light. The biggest study of these dark gamma-ray bursts to date, using the GROND instrument on the 2.2-metre MPG/ESO telescope at La Silla in Chile, has found that these gigantic explosions, while puzzling, don't require exotic explanations. Their faintness is now fully explained by a combination of causes with the most important being the presence of dust between the Earth and the explosion.  Credit: ESO/L. Calçada

This artist’s impression shows a dark gamma-ray burst in a star forming region. Gamma-ray bursts are among the most energetic events in the Universe, but some appear curiously faint in visible light. The biggest study of these dark gamma-ray bursts to date, using the GROND instrument on the 2.2-metre MPG/ESO telescope at La Silla in Chile, has found that these gigantic explosions, while puzzling, don't require exotic explanations. Their faintness is now fully explained by a combination of causes with the most important being the presence of dust between the Earth and the explosion. Credit: ESO/L. Calçada

“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,” says the study’s lead author Jochen Greiner from the Max-Planck Institute for Extraterrestrial Physics in Garching bei München, Germany.

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 could be studied. In the new study, astronomers combined Swift data with new observations made using GROND [2] — a dedicated gamma-ray burst follow-up observation instrument, which is attached to the 2.2-metre MPG/ESO telescope at La Silla in Chile. 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 very 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 [3].

The team used a range of data, including their own measurements from GROND, in addition to observations made by other large telescopes including the ESO Very Large Telescope, 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

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