News flash! A burst goes the distance.

After more than 25 years of debating whether gamma-ray bursts come from within our galaxy or from far beyond it, astronomers report the first proof that one of these energetic flashes originates outside the Milky Way.

The burst, dubbed GRB 970508, was observed May 8 by the Dutch-Italian satellite BeppoSAX and lasted only 35 seconds. Within a few hours, astronomers had found a visible-light counterpart--a starlike object fluctuating rapidly in brightness at the burst's position. Howard E. Bond of the Space Telescope Science Institute in Baltimore and his collaborators detected this afterglow from the Kitt Peak National Observatory near Tucson, Ariz., while Mark R. Metzger of the California Institute of Technology in Pasadena and his colleagues found it in observations from the Palomar Observatory near Escondido, Calif.

These findings rank as a milestone. Astronomers have only once before identified the visible-light counterpart of a gamma-ray burst; that burst was also detected by BeppoSAX (SN: 5/17/97, p. 305; 3/22/97, p. 174).

On May 11, Metzger and his collaborators made an even more momentous discovery. Using Keck II, the newer of the two 10-meter telescopes atop Hawaii's Mauna Kea, the astronomers measured spectra of the counterpart. The spectra revealed that light emitted by the object had passed through an intergalactic cloud, which absorbed certain wavelengths of the radiation. The wavelengths that the researchers detected indicate that the cloud lies some 7 billion light-years from Earth, about halfway to the edge of the observable universe. At this distance, the burst must be highly energetic to have been detected.

The finding that the visible-light afterglow, and therefore the burst itself, must lie beyond the cloud clinches the notion that this burst originated far from our galaxy. The astronomers don't know whether the counterpart resides immediately behind the cloud or much farther away, but other evidence from the spectra suggests it cannot be more than 12 billion light-years from Earth. Metzger and his colleagues report their findings in the May 11 circular of the International Astronomical Union.

Gamma-ray theorist Bohdan A. Paczynski of Princeton University says the data provide "proof that the distance to the burst is cosmological." He notes, however, that the jury is still out on whether other bursts are extragalactic.