Shedding more light on gamma-ray bursts.

Spitting out a torrent of radiation and then vanishing without a trace, gamma-ray bursts are among the most mysterious phenomena known in the universe. New findings from the Compton Gamma Ray Observatory (GRO), launched in 1991, may eventually help settle a debate about the origin of these bursts, says Bohdan Paczynski of Princeton University.

Last month, researchers announced that GRO had detected the highest-energy gamma-ray flash ever recorded (SN: 4/24/93, p.260). Brenda Dingus of NASA's Goddard Space Flight Center in Greenbelt, Md., says the finding suggests that gamma-ray bursts beam their energy rather than spewing it in all directions. If this interpretation proves correct, the bursts detected by GRO represent only a small fraction of the total number in the sky - those that happen to beam their radiation toward Earth. Dingus estimates that if gamma-ray bursts originate in a halo around our galaxy, they may be 100 times more numerous than indicated by GRO; if they originate far beyond our galaxy, they could be a million times more numerous.

Dingus says she favors the beam model because it would explain how the record-breaking burst recently detected could contain such a large number of high-energy gamma rays. High-energy gamma rays easily collide with lower-energy gamma rays, annihilating each other to produce pairs of subatomic particles. Relatively few high-energy photons are left to stream into space. However, if gamma-ray bursts beam their energy, most photons would travel in the same direction. Fewer head-on collisions would exist and more high-energy photons might reach Earth.

An analysis of the first 220 bursts recorded by GRO's Burst and Transient Source Experiment (BATSE) may offer indirect support for the beam model, says Chryssa Kouveliotou of NASA's Marshall Space Flight Center in Huntsville, Ala. She told SCIENCE NEWS that the unpublished results suggest these bursts fall into two classes: about 60 higher-energy flashes lasting less than 2 seconds and 160 lower-energy flashes that lasted longer. The two types, she notes, have the same intensity range.

Kouveliotou proposes that both classes stem from the same type of beamed source. She speculates that some bursts seem to have a shorter duration because only a portion of the beam is aimed directly at Earth. Thus, GRO would record only part of the event. She adds that BATSE has detected fewer faint bursts than expected; Paczynski says this offers more evidence that bursts originate outside our galaxy. The shortfall of faint bursts, he says, may indicate that bursts did not exist in the early universe or that faint bursts are so distant that their radiation has shifted to wavelengths longer than those of gamma rays.

The 600 bursts so far seen by BATSE, Kouveliotou notes, continue to show the same pattern - an even distribution across the sky. Because of Earth's off-center location in the Milky Way, she says, the symmetric distribution argues against a burst source associated with our own galaxy unless the bursts originate in a huge halo whose inner diameter is double that of the Milky Way. Jon E. Hakkila of Mankato (Minn.) State University and his colleagues calculate that if BATSE eventually records another 2,000 bursts that are evenly distributed, the halo required to match the data would be so large and uniform that scientists would have to abandon the halo theory.