Gamma-ray burst makes quite a bang.

For one brief moment, long ago in a faraway galaxy, a titanic explosion poured a torrent of gamma rays into space. Some 12 billion years later--Dec. 14, 1997--this flash of radiation reached Earth.

Astronomers are calling this gamma-ray burst "the most powerful explosion since the Big Bang." While that may be hyperbole, researchers have calculated that this cosmic flash packed 100 times more energy than a supernova explosion. Until now, researchers had considered supernovas the most energetic phenomenon known.

For the second or two that it lasted, "this burst was as luminous as all the rest of the entire universe," says S. George Djorgovski of the California Institute of Technology in Pasadena, a member of the team reporting the finding in the May 7 Nature. The group calculated the energy from the brightness of the burst and its afterglow, as well as the distance of the host galaxy from Earth--12 billion light-years.

Gamma rays from the burst were detected by the Dutch-Italian BeppoSAX satellite and NASA's Compton Gamma Ray Observatory. Then came a crucial step in finding the host galaxy. BeppoSAX also recorded an X-ray afterglow, part of the smoldering fireball that lingers after gamma rays have vanished.

A few hours later, using the afterglow as a guide, Jules P. Halpern of Columbia University and his colleagues detected a visible-light afterglow, they report in the May 7 Nature. Two weeks later, Djorgovski's team used the Keck II Telescope on Hawaii's Mauna Kea to find the host galaxy.

This marks the second time that astronomers have measured the distance to a galaxy that hosted a gamma-ray burst (SN: 5/17/97, p. 305).

These observations settle the long-standing debate over whether most gamma-ray bursts originate within our galaxy or far beyond it, some astronomers say. However, several of the findings call into question a popular theory in which bursts are generated when two dense stars, known as neutron stars, collide and merge.

Dale A. Frail of the National Radio Astronomy Observatory in Socorro, N.M., notes that to generate the energy associated with the Dec. 14 burst, virtually the entire mass of the neutron stars had to have been converted into gamma rays--an unlikely situation.

Frail told Science News that data from another burst, detected March 29, may prove equally damning for the theory. For the first time, researchers glimpsed an afterglow at radio wavelengths before finding one in visible light.

That sequence suggests that the burst originated from a place containing lots of dust, which blocks visible light but is transparent to radio waves. Stellar nurseries are rich in dust, and previous studies have hinted that several other bursts originated in star-forming locales. Neutron stars "cannot merge within star-forming regions," asserts Bohdan Paczynski of Princeton University. He explains that during the 100 million years or so that it would take for neutron stars to form and merge, they would have migrated from their birthplace.

Paczynski favors another model--described in the Feb. 10 Astrophysical Journal Letters--in in which a massive, short-lived star undergoes a "hypernova" explosion, hurling a shock wave into space at nearly the speed of light.