Birth zone shrinks for top cosmic rays.

When cosmic rays, the universe's most energetic particles, pound into Earth's atmosphere, they unleash showers of particles streaking down with a dim, blue glow. Of these rays, those with the highest energy pepper the planet with the densest sprays over the widest areas, up to 10 kilometers across.

Astrophysicists have long pondered where and how the strongest cosmic rays acquire millions of times the energy that any earthbound particle accelerator can produce. Proposals on a growing list range from decaying superheavy particles in this galaxy to colliding cosmic strings of energy stretching across all of space.

If cosmic rays come from the farthest reaches of the universe, they must travel through the pervasive background radiation left over from the Big Bang. Theorists have shown that collisions with background-radiation photons would sap the cosmic rays' energies, limiting them to about 50 million trillion electronvolts (eV) on arrival at terrestrial detectors.

Yet cosmic rays are regularly, if infrequently, reaching Earth with higher energies, according to a report in the Aug. 10 Physical Review Letters. Led by Masahiro Takeda of the University of Tokyo, a 28-scientist team recorded six such major impacts from February 1990 to October 1997. If cosmic rays spring up throughout the universe, the researchers calculate they should have detected at most one. They used Japan's Akeno Giant Air Shower Array, the world's largest detector of subatomic particle sprays from cosmic rays.

Looking skyward to where the rays seemed to originate, the team found no astronomical objects potent enough to spit out such energetic particles. "[They] have to come from very nearby. But the things we see and think we understand nearby don't seem capable of making things of this energy," says physicist James W. Cronin of the University of Chicago.

The Japanese team concludes that the highest-energy cosmic rays must originate within 150 million light years of Earth. That region still contains plenty of possible candidate sources for the rays, Cronin says, but all require either new physics or a revised understanding of known celestial objects.

A slim chance persists that the cosmic rays come from afar, he adds, but few astrophysicists fault the energy-sapping theory or think that the most energetic cosmic rays are exotic particles whose energy wouldn't be sapped.

The new findings complement the 1993 detection of the most energetic--300 million trillion eV--cosmic ray ever recorded (SN: 12/4/93, p. 372). They sound a death knell for theories of distant origins for the most potent cosmic rays, says Alexander Vilenkin of Tufts University in Medford, Mass. He favors the idea that undiscovered superheavy particles around the galaxy, as they decay, eject the strong cosmic rays.

New experiments "will solve the puzzle of the highest energy cosmic rays," the Japanese team predicts. The Pierre Auger Project, for example, is a $100-million, 19-nation collaboration intended to erect two vast detectors in Argentina and Utah. On July 24, the U.S. government agreed to provide 15 percent of the funding for the Argentinean site, beginning in October. U.S. support for the rest of the project may come later, project backers say.

According to Cronin, a spokesman for the project, the 3,000-square-kilometer array of detectors in Argentina is designed to pick up 30 of the highest energy cosmic rays each year, improving scientists' ability to draw a bead on cosmic ray sources.