Light dawns on dark gamma-ray bursts (w/ Video)

Dec 16, 2010
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

( -- 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 so-called dark gamma-ray bursts, using the GROND instrument on the 2.2-metre 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 the Earth and the explosion.

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, 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. 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 video is not supported by your browser at this time.
This video podcast explains the ESO Very Large Telescope’s Rapid Response Mode, which makes it possible to observe gamma-ray bursts only a few minutes after they are first spotted. Credit: ESO

"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 ," 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 — a dedicated 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.

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󈞼 percent of the original intensity by obscuring dust, this effect is exaggerated for the very distant bursts, letting the observer see only 30󈞞 percent of the light. The astronomers conclude that most dark gamma-ray bursts are therefore simply those that have had their small amount of 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.

Explore further: How baryon acoustic oscillation reveals the expansion of the universe

Related Stories

GROND takes off

Jul 09, 2007

A new instrument has seen First Light at the ESO La Silla Observatory. Equipping the 2.2-m MPI/ESO telescope, GROND takes images simultaneously in seven colours. It will be mostly used to determine distances of gamma-ray ...

Keck Study Sheds New Light on "Dark" Gamma-ray Bursts

Jun 08, 2009

Since its launch in 2004, NASA's Swift has detected more than 430 gamma-ray bursts. Roughly half of them are "dark" bursts that emit little or no visible light. Dense knots of dust in otherwise normal galaxies ...

Tracking the Riddle of Cosmic Gamma Rays

Aug 23, 2005

First simultaneous observation of a gamma-ray burst in the X-ray and in the very high energy gamma ray band. For the first time a gamma-ray burst (GRB) has been observed simultaneously in the X-ray and in the ...

Controlled by distant explosions

Mar 28, 2007

At 11:08 pm on 17 April 2006, an alarm rang in the Control Room of ESO's Very Large Telescope on Paranal, Chile. Fortunately, it did not announce any catastrophe on the mountain, nor with one of the world's largest telescopes. ...

Swift Satellite records early phase of gamma ray burst

Mar 02, 2009

( -- UK astronomers, using a telescope aboard the NASA Swift Satellite, have captured information from the early stages of a gamma ray burst - the most violent and luminous explosions occurring ...

Worldwide hunt to solve the mystery of gamma-ray bursts

Feb 16, 2008

UK space scientist Emeritus Professor Alan Wells is to speak at the American Association for the Advancement of Science (AAAS) in Boston in February on International Cooperation in Developing Swift and its Scientific Achievements.

Recommended for you

Gravitational waves according to Planck

9 hours ago

Scientists of the Planck collaboration, and in particular the Trieste team, have conducted a series of in-depth checks on the discovery recently publicized by the Antarctic Observatory, which announced last spring that it ...

Infant solar system shows signs of windy weather

9 hours ago

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have observed what may be the first-ever signs of windy weather around a T Tauri star, an infant analog of our own Sun. This may help ...

Finding hints of gravitational waves in the stars

15 hours ago

Scientists have shown how gravitational waves—invisible ripples in the fabric of space and time that propagate through the universe—might be "seen" by looking at the stars. The new model proposes that ...

How gamma ray telescopes work

16 hours ago

Yesterday I talked about the detection of gamma ray bursts, intense blasts of gamma rays that occasionally appear in distant galaxies. Gamma ray bursts were only detected when gamma ray satellites were put ...

The frequency of high-energy gamma ray bursts

18 hours ago

In the 1960s a series of satellites were built as part of Project Vela.  Project Vela was intended to detect violations of the 1963 ban on above ground testing of nuclear weapons.  The Vela satellites were ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Dec 16, 2010
Very intersting, the using of satellites and automated telescopes ;D