European Southern Observatory | 16 Dec 2010
What's Hiding the Universe's Brightest Explosions?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, 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. 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.
“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 the Gamma-Ray burst Optical and Near-infrared Detector (GROND) — 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.
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.
Science Now | 16 Dec 2010
The nature of "dark" gamma-ray bursts - J Greiner et alSome of the most powerful explosions in the universe are all but invisible to even the largest telescopes on Earth. Astronomers have long wondered why they can't see these so-called dark-bursts. The answer, it turns out, is surprisingly simple.
When massive, rapidly rotating stars collapse into black holes at the end of their lives, they produce a seething fireball and brief explosions of extremely energetic radiation known as gamma-ray bursts. As the fireball cools, it emits a full spectrum of all kinds of radiation, from energetic x-rays to low-energy radio waves. But only about half of these burst afterglows give off visible light. The rest remain hidden to optical telescopes.
Astronomers have proposed two possible explanations for these dark bursts. Intervening dust might block the afterglow's optical light, whereas dust is transparent to x-rays, infrared radiation, and radio waves. Alternatively, hydrogen atoms might be the culprit. Neutral hydrogen absorbs certain wavelengths of ultraviolet light, so burst radiation passing through a hydrogen cloud would wind up with a gap in the ultraviolet part of its spectrum. But if a burst is extremely remote, its radiation travels through the expanding universe for billions of years. As a result, the full spectrum, including the gap, gets stretched to longer and longer wavelengths, a phenomenon known as the "red shift." Eventually, the absorption gap may end up at optical wavelengths by the time the radiation reaches Earth.
To find out which of the two explanations plays the largest role, an international team of astronomers led by Jochen Greiner of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, studied 39 gamma-ray burst afterglows using a dedicated instrument at the European La Silla Paranal Observatory in northern Chile.
Within minutes after NASA's SWIFT satellite detects a new gamma-ray burst, a 2.2-meter telescope at La Silla, owned by the Max Planck Institute and the European Southern Observatory, is aimed at the burst's sky position, and the afterglow is monitored at 7 wavelengths by the Gamma-Ray Burst Optical/Near-Infrared Detector.
As expected, many of the observed afterglows turned out to be extremely dim or even invisible at optical wavelengths. But by studying these dark bursts at various wavelengths simultaneously, Greiner and his team found that absorption by dust is the only viable explanation in most cases. The alternative—redshifted hydrogen absorption—would produce a different energy distribution across the observed spectrum; it explains only a handful of dark bursts.
"This really answers the question for the first time," says astrophysicist Neil Gehrels of NASA's Goddard Space Flight Center in Greenbelt, Maryland, who is the principal investigator of NASA's SWIFT satellite. The large amount of light extinction by dust, he adds, indicates that gamma-ray bursts occur in the densest, dustiest regions of the universe.
Indeed, says astronomer Johan Fynbo of the University of Copenhagen, gamma-ray bursts can be used as a tool to study star formation throughout the history of the universe. Because the most massive stars have very short life spans, gamma-ray bursts still probe the regions where new stars are born. "We want to know which percentage of star formation occurs in very dusty environments," he says.
Fynbo agrees that the new results, published today in Astronomy & Astrophysics, firmly establish that dust extinction is the dominant reason so many gamma-ray bursts are dark. "It's pretty well settled." But he says more work is needed to find out whether the obscuring dust is mostly spread throughout the host galaxy, or whether most of the absorption takes place close to the burst.
- Astronomy and Astrophysics 526 A30 (Feb 2011)
(online 16 Dec 2010) DOI: 10.1051/0004-6361/201015458
arXiv.org > astro-ph > arXiv:1011.0618 > 02 Nov 2010