GIANT COSMIC EXPLOSION ASTOUNDS ASTRONOMERS.

Byline: Malcolm W. Brown The New York Times

Astronomers have detected a titanic explosion in the outer reaches of the cosmos - one so violent and bright that for about 40 seconds it appeared to outshine all the rest of the universe. Except for the Big Bang that apparently created the universe, no other cosmic explosion of such magnitude has ever been measured.

The observations that led to this estimate caught theorists completely off guard, Dr. John N. Bahcall of the Institute for Advanced Study at Princeton, said Wednesday. Either new observations of similar explosions will lower the estimated energy output, or theorists will be forced to seek some entirely new explanation for the stupendous outpouring of energy.

``I'm a very troubled theorist,'' said Dr. Stanford E. Woosley of the University of California at Santa Cruz. ``We're really struggling to find a theoretical basis for this.''

The results of an investigation of the explosion by many scientific institutions in the United States, Europe and Asia were announced at a news conference held Wednesday in Washington by the National Aeronautics and Space Administration and are being published in papers today by the journal Nature.

The 12-billion-year-old event that brought on these frantic international studies was detected nearly five months ago. On Dec. 14, at 6:34 p.m., Eastern standard time, an exquisitely sensitive orbiting gamma-ray observatory called BeppoSAX, which was built by an Italian-Dutch collaboration of astronomers and launched two years ago, signaled to its operators that it had recorded something interesting. For about 40 seconds, the satellite measured a sharp pulse of gamma rays. More important, it pinpointed the position of the rays' source in the sky.

The Dec. 14 pulse was also detected by the United States' Compton Gamma Ray Observatory satellite. As measured by the American satellite, the burst's gamma-ray brightness appeared fairly typical of gamma-ray bursts in general. But what astronomers did not realize at that point was that the event had occurred almost incredibly far away, and must therefore have been immensely powerful to look so bright from Earth.

Thousands of gamma-ray ``bursters,'' as such events are called, have been detected since the 1960s, but they remain mysterious, because each one disappears in seconds or minutes and has not been known to recur for further study. Except for the Dec. 14 burster, none of their distances from Earth have ever been determined.

But this burster, designated GRB971214, gave astronomers some of the clues they had long sought.

Right after the gamma-ray burst was detected, dozens of scientific institutions in many parts of the world raced to look for any traces it might have left. Gamma rays are the most energetic of all forms of electromagnetic radiation, but as an object emitting them cools, it radiates waves of lower energy: X-rays, ultraviolet radiation, visible light, infrared rays and, at the lowest range of the energy spectrum, radio.

Since BeppoSAX was launched in 1996, its unmatched ability to detect the exact location of a fleeting gamma-ray burst has made it possible to look for the lingering afterglow of a burst, which can take the form of X-rays, visible light and other types of radiation.

Twelve hours after the December burst occurred, Dr. John R. Thorstensen of Dartmouth College, using a 94-inch-diameter telescope at the Kitt Peak Observatory in Arizona, found a visible afterglow.

This visible light persisted for about two weeks, and then something even more interesting turned up at the same spot in the sky: scientists found a faint galaxy.

Astronomers realized that this galaxy was probably the host of the gamma-ray explosion and that its distance from Earth must therefore be the same as that of the burster. Many large telescopes were brought to bear on the galaxy, and the Keck II in Hawaii, one of the two largest telescopes ever built, hit pay dirt. A team of Amraerican, Italian and Indian astronomers led by Dr. Shrinivas R. Kulkarni and S. George Djorgovski, both of the California Institute of Technology, managed to measure the distance to the burster.

Kulkarni said Wednesday that the distant galaxy that spawned the burster appeared to be creating a myriad of new stars, and that the burst might in some way be associated with this process.

CAPTION(S):

Photo

PHOTO (color) This photo, taken by the Hubble Space Telescope, shows the largest explosion ever seen.

Associated Press