Turning a fly's eye on energetic cosmic rays.

The origin of high-energy cosmic rays has long puzzled astrophysicists. Were these electrically charged particles accelerated to extremely high velocities outside the Milky Way or in turbulent, supernova-disturbed regions within our galaxy?

Using two ground-based Fly's Eye detectors to pick up the faint streaks created in the night sky by the passage of energetic cosmic rays plunging through Earth's atmosphere, researchers have now obtained the clearest evidence yet that cosmic rays of the highest energies detectable consist largely of protons that apparently originated outside the Milky Way. Cosmic rays of somewhat lower energy consist mainly of atomic nuclei of such heavy elements as iron. These rays probably originated within our galaxy.

The results reveal a "dramatic transition" from one type of cosmic ray to another at an energy between 1018 and 1019 electron-volts, Eugene C. Loh of the University of Utah in Salt Lake City and his collaborators report in the Nov. 22 PHYSICAL REVIEW LETTERS. This finding provides clues that may help determine where and how these particles are accelerated to such high energies.

"This paper is extremely interesting," comments Gary P. Zank of the Bartol Research Institute at the University of Delaware in Newark. "The suggestion, made over the years, that the very-high-energy particles are extragalactic in origin is probably nailed down fairly well."

Cosmic rays pierce Earth's atmosphere with sufficient energy to leave a cascade of charged particles in their wake. Because these particles excite nitrogen molecules, the air along these paths through the atmosphere glows with a dim blue light.

The Fly's Eye detector consists of a collection of mirrors and photomultiplier tubes packed together to look like a fly's compound eye. This arrangement allows researchers to monitor the entire night sky for cosmic-ray tracks. Using two such detectors, located 3.4 kilometers apart in the desert near Dugway. Utah, they can deduce the energy of an incoming cosmic ray and determine its arrival direction.

Observing the sky for more than a decade, the Fly's Eye team has accumulated sufficient data to produce a spectrum showing the intensity of cosmic rays at different energies. The researchers interpret an obvious "dip" in the plotted spectrum as evidence that high-energy cosmic rays come in two distinct varieties, with different origins.

The detection of a cosmic ray with an energy of 3 x 102o electron-volts - the highest energy ever recorded for a cosmic ray - also suggests that these rays can't be relics from the early universe. Because a cosmic ray loses energy through interactions with the background microwave radiation that permeates the universe, this particular ray must have come from a source less than 100 million light-years away.

The new data may help theorists decide whether highly energetic cosmic rays originate in the dense nuclei of galaxies, where tremendous concentrations of stars or even black holes can create environments in which protons can be accelerated to high velocities. The lower-energy findings focus attention on exactly what happens inside supernova remnants to accelerate heavy nuclei