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>  News Releases >   1998 >   May

Researchers describe biggest cosmic explosion since the Big Bang

Posted 05/07/98

A violent explosion detected several months ago from deep in the cosmos may have been the most powerful eruption ever witnessed, except for the Big Bang which scientists think created the entire observable universe. The violent cosmic event is described in detail in today's issue of the journal Nature. "This is a long-standing cosmic mystery," says John Thorstensen, professor of physics and astronomy at Dartmouth and a co-author of one of the studies. "There is a tremendous amount of energy associated with such an event - but nobody knows what its source is."

The blast of high-energy radiation, known as a gamma-ray burst, lasted about a second, releasing almost as much energy as the 10 billion trillion stars in the universe combined. The event is reported by an international team of 16 astronomers. Thorstensen, who photographed the event using an imaging camera on a telescope at Kitt Peak, Ariz., is a member of a four-strong rapid-response team that supplied details of its afterglow in the constellation Ursa Major.

The burst was first detected on Dec. 14, 1997, by the Italian/Dutch BeppoSAX satellite and NASA's Compton Gamma Ray Observatory satellite. The Compton observatory provided detailed measurements of the total brightness of the burst, while BeppoSAX provided its precise location, enabling follow-up observations with ground-based telescopes and NASA's Hubble Space Telescope.

Shortly after the burst was detected, a team of astronomers from the California Institute of Technology in Pasadena measured the distance between Earth and the faint galaxy from which the burst originated at about 12 billion light years. "To visualize what that means - if the universe were shrunken so greatly that the sun was the size of an atom, the distance to the burst would be from Hanover to Hawaii," says Thorstensen.

Astronomers do not customarily use imaging cameras on nights when there is a bright moon, since moonlight makes dimmer stars difficult to see. But Thorstensen was at Kitt Peak getting images of bright stars, so when colleagues from Columbia called to report the burst and its coordinates, he was able to photograph the region of the gamma-ray burst within 12 hours of the satellite detection, using a telescope fitted with an electronic camera that produces images instantly.

"I processed the images right away, but when I compared them to survey images on the Web, I couldn't see any evidence of new stars." He reported disappointment to his colleagues, who suggested taking additional pictures in 24 hours. At 5 the next morning he did, comparing the new images to the previous series using a program that rapidly alternates between two images, so that any change appears to wink on and off. "Within five seconds, bam, there it was." He dialed his colleagues to report, "We got it!"

Thorstensen had found that a faint star lurking on the first night's image had faded to near-invisibility on the second night, when the burst was older. He set to measuring its precise position, while his Columbia colleagues composed a quick announcement. The news from Kitt Peak prompted colleagues in other locations to zero in on these coordinates for a more detailed study of the explosion.

Gamma-ray bursts are mysterious flashes of high-energy radiation that appear from random directions in space and typically last a few seconds. They were first discovered in the 1960s by U.S. Air Force satellites. Since then, numerous theories of their origin have been proposed, but the causes of gamma-ray bursts remain unknown.

Until recently, the principal limitation in understanding these bursts was the difficulty in pinpointing their direction on the sky. Unlike visible light, gamma rays are exceedingly difficult to observe with a telescope, and the bursts' short duration exacerbates the problem. With BeppoSAX, scientists now have a tool to localize the bursts on the celestial sphere with sufficient precision to permit follow-up observations with the world's most powerful ground-based telescopes.

This led to the discovery of long-lived "afterglows" of bursts in X-rays, visible and infrared light, and radio waves. While gamma-ray bursts last only a few seconds, their afterglows can be studied for several months. Study of the afterglows indicated that the bursts do not originate within our own galaxy, the Milky Way, but rather are associated with extremely distant galaxies.

From the distance and the observed brightness of the burst, astronomers derived the amount of energy released in the flash. Scientists say it is possible that other forms of radiation from the burst, such as neutrinos or gravity waves, which are extremely difficult to detect, carried a hundred times more energy than that.

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