Gamma Blast From the Past

COLUMBUS, OHIO--A blast of gamma rays picked up by satellites last December originated 10 billion years ago at the very edge of the visible universe, observers reported here last Sunday. The ancient event was a gamma ray burst, one of the flashes of high-energy radiation that astronomers think originate in the most energetic events in the universe. The spectacular distance of this event could force theorists to come up with still more energy to stoke the mysterious bursts.

At a meeting of the American Physical Society, Shrinivas Kulkarni of the California Institute of Technology described how he, his Caltech colleague George Djorgovski, and others used the 10-meter Keck Telescope in Hawaii and NASA's Hubble Space Telescope to study the site of a 14 December gamma ray burst. By making a spectrum of a fuzzy patch, presumably the galaxy in which the original explosion took place, Kulkarni and Djorgovski were able to measure its "redshift"--the stretching of its light by the universe's expansion, which is a measure of distance. The result, detailed in a paper accepted at Nature, is a redshift of 3.418. That puts the source at a distance several times greater than the only comparable measurement, a figure of 0.835 or more for another burst's optical counterpart (Science, 23 May 1997, p. 1194).

A cascade of observations at different wavelengths led to the discovery. Gamma ray detectors can't accurately pin down the location of a burst, but the cameras aboard the Italian-Dutch BeppoSAX satellite spotted the x-ray afterglow of the 14 December event and determined a relatively accurate position for it. That allowed other astronomers to find a fading optical counterpart. Once the fading leveled off--suggesting that the light of the host galaxy was emerging--Kulkarni captured the galaxy's spectrum and measured its redshift.

"It's just spectacular," says Princeton University astronomer Neta Bahcall. "This is almost at the fringes of anything observed in the universe," says Jonathan Katz, an astrophysicist at Washington University in St. Louis. Many theorists believe that a gamma ray burst is the first flash from a violent fireball, perhaps triggered by the collision of two neutron stars. The titanic energy implied by this latest measurement could send theorists looking for ways to squeeze more brilliance from the fireball, perhaps by focusing its energy with magnetic fields.