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 VOL. 23, NO. 23MAY 20, 1998 


Columbia Astronomers Report Biggest Explosion Ever Observed

Jules Halpern
 
David Helfand


 BY BOB NELSON

Thanks to the quick action of an “astronomical SWAT team” at Columbia, scientists have detected the largest explosion in the universe.

  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, 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 American satellites sent into Earth orbit to verify the Comprehensive 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 at a NASA press conference May 6 in Washington and was published in three papers in the May 7 issue of the British journal Nature by astronomers at Caltech, Columbia and several other institutions. Labeled GRB971214 after Dec. 14, the date it was observed, it is the third-ever optical detection of a gamma-ray burst.

  Gamma rays are invisible, extremely short wavelength electromagnetic radiation, much like light or radio waves, but at much higher energies. GRB 971214 was first detected by the X-ray observatory aboard an Italian-Dutch satellite, BeppoSAX.

  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.

  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. 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,” Halpern said. “We think of ourselves as an astronomical SWAT team.”

  Within two weeks, a Caltech team led by Shrinivas Kulkarni detected an extremely faint galaxy at the location, using one of the world’s largest telescopes, the W. M. Keck Observatory’s 10-meter Keck II telescope, on Mauna Kea, Hawaii. Subsequent images taken with NASA’s Hubble Space Telescope by the Columbia-Caltech team confirmed that the burst’s afterglow was located in this faint galaxy.

  The observation might 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 larger than the Sun, into a rotating black hole. But 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, 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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