Contact: Bob Nelson
(212) 854-6580
Embargoed for release
2 P.M. EDT, May 6, 1998

Note to editors: The new gamma-ray burst, GRB971214, will be the subject of a NASA press conference May 6 at 2 P.M. EDT in the NASA Headquarters Auditorium, Washington, D.C. David J. Helfand, professor of astronomy at Columbia, will participate. He is also available at his office, (212) 854-2150, or home, (212) 663-5421.

Columbia Astronomers Detect Biggest Explosion Ever Observed

Theorists At Loss to Explain What Enormous Blast Could Have Been; Perhaps Supermassive Star Collapsed, Forming Black Hole

Thanks to the quick action of an "astronomical SWAT team" at Columbia University, scientists today reported that they had detected the largest explosion ever witnessed.

The explosion, a blast of high-energy radiation known as a gamma-ray burst, was found in the constellation Ursa Major and lasted about a second last Dec. 14, releasing almost as much energy as the 10 billion trillion stars in the universe combined. A rapid series of phone calls between astronomers around the world brought powerful telescopes to focus on the event, which occurred 12 billion years ago when the universe was relatively young.

Gamma-ray bursts were first detected in 1967 by the Vela satellites, sent into Earth orbit to verify the Nuclear Test Ban Treaty. More recently, using data obtained since 1991 by the Compton Gamma-Ray Observatory, a NASA satellite, scientists began to suspect that these bursts, which occur from random points in the sky, might be coming from far beyond the confines of our own Milky Way galaxy. But what causes them remains a mystery.

The observation was discussed today at a NASA press conference in Washington, D.C., and is published in three papers in the May 7 issue of the British journal Nature by astronomers at the California Institute of Technology, Columbia and several other institutions. Labeled GRB971214 after the date it was observed, it is the third-ever optical detection of a gamma-ray burst.

Optical data are needed to locate distant events such as stellar explosions. The expanding universe shifts light from distant stars toward the red end of the visible spectrum; the larger this red shift, the more distant the object. The Caltech team found a red shift of 3.42, an extraordinarily large number that indicated the light emitted by GBR971214 took 12 billion years to travel to earth, 80 percent of the most-widely accepted age of the universe, 15 billion years. The event thus took place some 8 billion years before the EarthÕs formation.

Gamma rays are invisible, extremely short wavelength electromagnetic radiation, much like light or radio waves, but at much higher energies. They are absorbed by the atmosphere, which is why they were unknown until satellite detectors were sent aloft. And until recent optical observations, gamma rays were the only signature of the explosions known as gamma-ray bursts; thus their name. Gamma-ray bursts are also very difficult to locate, both because they cannot be resolved into an image and because they are often very faint and distant.

GRB971214 was first detected Dec. 14 by the X-ray observatory aboard an Italian-Dutch satellite, BeppoSAX, and then by the Compton Gamma-Ray Observatory. An Italian astronomer, Enrico Costa of the Instituto di Astrofisica Spaziale in Frascati, who is part of the BeppoSAX research team, telephoned David J. Helfand, professor of astronomy at Columbia, where it was 11:15 P.M. on a Sunday evening, and notified him of the approximate location of the event.

Professor Helfand relayed the message to a Dartmouth astronomer, John Thorstensen, who was viewing the sky using the 2.4-meter telescope at the MDM Observatory on Kitt Peak, near Tucson, Ariz., a facility jointly owned and operated by Columbia, Dartmouth, the University of Michigan and Ohio State University. Professor Thorstensen was able to photograph the region of the gamma-ray burst within 12 hours of the satellite detection, and the next night identified the object, which was now noticeably fainter, establishing that it was the optical afterglow of the gamma-ray burst. Jules Halpern, professor of astronomy at Columbia, interpreted the results.

"You need luck, persistence, quick response and quick thinking to catch these events," Professor Halpern said. "We think of ourselves as an astronomical SWAT team."

The Columbia report was confirmed by the 3.5-meter telescope at the Apache Point Observatory nearby and was reported to the astronomical community through the Central Bureau for Astronomical Telegrams. Within two weeks, a Caltech team led by Shrinivas Kulkarni detected an extremely faint galaxy at the location, using one of the worldÕs largest telescopes, the W. M. Keck ObservatoryÕs 10-meter Keck II telescope, on Mauna Kea, Hawaii. Subsequent images taken with NASAÕs Hubble Space Telescope by the Columbia-Caltech team confirmed that the burstÕs afterglow was located in this faint galaxy.

At the time of the viewing, the leading hypothesis for the cause of gamma-ray bursts was that two neutron stars spiraled into each other and exploded. But neutron stars are very old stars, and the dusty location where the explosion took place is a region of stellar birth, not old age. And even an exploding star, or supernova, until now the most energetic event observed in the universe, is only a hundredth as powerful as this gamma-ray burst.

The observation might, however, be consistent with a so-called "hypernova," a term coined by Princeton astronomer Bohdan Paczynski to denote the collapse of an extraordinarily large star, one 80 to 100 times as large as the Sun, into a rotating black hole. Since stars of that size live only 3 million years, they never get far from where they are born and would be seen in such a region.

"We speculate that when any star that big runs out of nuclear fuel, it will collapse into its core, as do all stars with masses more than ten times that of the Sun," Professor Helfand said. "Stars with that much more material to rain down into its center could crush matter beyond making a normal neutron star and create a black hole. We of course have no evidence of this so far."

But Professors Halpern and Helfand said that the titanic explosion could well involve phenomena never before theorized. To work out the actual mechanisms involved, scientists need more cases to study, and the gamma-ray satellites now in orbit can only locate about a half-dozen a year, Professor Helfand said. Columbia and Caltech scientists are hoping to build and launch a gamma-ray observatory that would provide more accurate positions of hundreds of such events, allowing astronomers to look at them with ground-based telescopes before they become too faint to see.

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