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MPE News December 16, 2010

Illuminating dark bursts with GROND

Gamma-ray bursts are among the most energetic events in the Universe, but some appear curiously faint in visible light. An international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics have now conducted the biggest study to date of these so-called dark gamma-ray bursts, using the GROND instrument on the 2.2-metre MPG/ESO telescope at La Silla in Chile. The scientists conclude that these gigantic explosions do not 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 the Earth and the explosion.

Gamma-ray burst
Dust in the surrounding of a GRB will dim and redden the light before it reaches the observer.
Credit: MPE / J. Greiner
Gamma-ray bursts (GRBs), named after the flare of highly energetic light which they emit for a very short period of time, from less than a second to several minutes, are detected by orbiting observatories. 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 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.

"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.

The NASA Swift satellite can detect gamma-ray bursts from its orbit above the Earth's atmosphere and immediately relay their positions to other observatories. In the new study, astronomers combined Swift data with new observations made using GROND, the Gamma-Ray burst Optical and Near-infrared Detector, which was designed and built at the Max-Planck Institute for Extraterrestrial Physics in collaboration with the Thüringer Landessternwarte Tautenburg and has been fully operational at the 2.2-metre MPG/ESO telescope at La Silla in Chile since August 2007.

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.

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 60-80 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," says Greiner.

Original paper :
  The nature of “dark” gamma-ray bursts
J. Greiner, T. Krühler, S. Klose, P. Afonso, C. Clemens, R. Filgas, D. H. Hartmann, A. Küpcü Yoldas, M. Nardini, F. Olivares E., A. Rau, A. Rossi, P. Schady and A. Updike
external link A&A 526, A30 (2011)

Press release :
  external link ESO press release

Links :
  internal link GROND Webpages at MPE
internal link GROND Pictures at MPE (with explanations in German)

Contact :
  internal link Dr. Jochen Greiner
Max-Planck-Institut für extraterrestrische Physik
phone: +49 89 30000-3847
email: jcg@mpe.mpg.de
  internal link Dr. Hannelore Hämmerle
Press Officer
Max-Planck-Institut für extraterrestrische Physik
phone: +49 89 30000-3980
email: hanneh@mpe.mpg.de
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