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Massive Cosmic Explosion Has Astronomers Stumped

Brian Handwerk
for National Geographic News
December 20, 2006
 
An unusual gamma-ray burst has astronomers wondering what new type of
cosmic explosion could have created the brilliant blast of light.

Gamma-ray bursts are brief explosions of super-energetic light at the most extreme range of the electromagnetic spectrum.

A single burst is a million trillion times brighter than the sun. It lasts only a short time—from a few milliseconds to a few minutes.

"They are the most violent events known in our universe," said Bing Zhang of the University of Nevada, Las Vegas.

"There are a lot of unknowns, but their study tells us about the extreme conditions found in the universe."

The bursts appear to be associated with supernovas—the massive explosions caused by the deaths of some stars that can spawn black holes.

But a gamma-ray burst observed by NASA's Swift satellite on June 14, 2006, defies any currently known theories, because it reveals no evidence of an associated supernova.

"The fact that this one didn't [associate with a supernova] is making us rethink our whole idea of what can cause gamma-ray bursts," said Neil Gehrels of the NASA Goddard Spaceflight Center in Greenbelt, Maryland.

Gehrels authored one of four studies of the mysterious burst that appear in tomorrow's issue of the journal Nature.

Explosion Defies Characterization

The prevailing theory is that a catastrophic explosion in the core of a star collapsing into a black hole launches a shock wave of energy that sends gamma rays surging from the star's surface.

(See images of a gamma-ray burst taken by the Hubble Space Telescope.)

"We've learned over the last ten years that gamma-ray bursts are caused by the birth of a black hole," Gehrels said.

"They are kind of the birth cries of black holes, so they are a way to study how the universe produces black holes and how stars end their lives."

Shorter bursts, those lasting less than two seconds, are thought to have their origins in different types of explosions. They may result from a massive collision between two neutron stars (other kinds of dense remnants formed by collapsing stars) or the meeting of a single neutron star and a black hole.

But the burst currently under investigation, dubbed GRB 060614, lasted for 102 seconds, much too long to be one of these short bursts.

A second burst has recently been observed with similarly puzzling properties, suggesting that the first was not an anomaly.

The two strange explosions share many characteristics of short-term bursts, except for their duration.

"Maybe it's time to not call these [bursts] short and long but just type one and type two. It could also be a third type—at the moment we don't know," said Nevada's Zhang, who also contributed an article to the Nature report.

"Bursts like this are rare, but we suddenly got two of them, which means that there's a good chance that Swift will see a handful of them in the next few years."

In the meantime, some interesting theories have been proposed to explain the violent explosion.

"If you have a black hole that was made by some previous [supernova], and it collided with a neutron star, it could tear up the neutron star and produce a longer burst. That's one kind of wild idea for this particular event," Gehrels said.

Catastrophe for Earth

Gamma-ray bursts are observed about once a day and pack about as much energy as would be released during the entire life spans of a thousand suns.

The bursts originate in galaxies billions of light-years from Earth, so the bursts we see today actually occurred when Earth was newly formed and the universe was much younger.

(See a National Geographic magazine feature on the Hubble Space Telescope: "Eye on Infinity.")

Gamma rays are powerful enough to kill living cells, but those emitted in space don't reach Earth because they are absorbed by the atmosphere.

A particularly large gamma-ray burst could cause satellite communications problems in the upper atmosphere.

But if a burst ever occurred within our own galaxy, it could destroy Earth's ozone layer.

Fortunately, Gehrels reports, the chances of such an event are extremely remote.

"It may be that there has only been one [burst] over the past billion years [that was] close enough in the galaxy to have a significant effect on Earth," he said.

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