Some 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.