Scoping out the homes of gamma-ray bursts.

In a matter of seconds, gamma-ray bursts unleash more energy than the sun has released during the past 4.5 billion years. Astronomers don't know what generates these flashes of energetic radiation, but they have begun to identify the galaxies from which the bursts originate.

Earlier this month, astronomers succeeded in measuring the distance to the galaxy from which a gamma-ray burst detected last June 13 by the BeppoSAX satellite had emerged. The measurement, based on observations with the Keck II Telescope atop Hawaii's Mauna Kea, marks only the fourth time that researchers have measured the distance to the source galaxy of a gamma-ray burst.

Shrinivas R. Kulkarni, S. George Djorgovski, and Joshua S. Bloom of the California Institute of Technology in Pasadena and their colleagues reported the finding in a Jan. 3 circular of the Gamma-Ray Burst Coordinates Network (GCN). The galaxy lies about 6 billion light-years from Earth.

Researchers have also explored the origins of a gamma-ray burst recorded by several satellites on Dec. 20, 1998. The astronomers found what appear to be the fading visible-light ember of the burst and the faint light of its home galaxy. In a Dec. 29, 1998, GCN circular, Aaron C. Eichelberger of Caltech and his colleagues were the first to report the visible glow associated with the burst. Other teams also recorded images of the ember and its home galaxy, which coincide with the position of a radio-wavelength afterglow to the gamma-ray burst. Djorgovski and his collaborators took a detailed visible-light image Jan. 14 with the Keck I Telescope on Mauna Kea.

"At this point it is simply a feat [to detect] the thing: Its brightness is about equal to that of a 40-watt light bulb seen from 1 million miles away!" says Djorgovski. The galaxy's faintness suggests it is distant, he adds.

Like light bulbs of different wattages, the bursts come in a wide range of luminosities, Kulkarni notes. In the Feb. 1 Astrophysical Journal Letters, Bradley E. Schaefer of Yale University argues that many of the galaxies known to have produced gamma-ray bursts are dimmer than the average galaxy. There is no clear explanation for the finding.

Despite such puzzles, "slowly but surely, we are starting to understand the population of the host galaxies of gamma-ray bursts," says Djorgovski. The researchers hope that such understanding will shed light on the processes that create these energetic outbursts.