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University of Chicago News Office
Chicago, Illinois

Contact:
Steve Koppes, (773) 702-8366, s-koppes{at}uchicago.edu

Feb. 16, 2003

Auger experiment could resolve mystery of high-energy cosmic rays
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Some mysterious source from beyond the galaxy periodically zaps Earth
with high-energy cosmic rays from all directions. So far, the two
leading experiments devoted to the detection and understanding of
these rays have yielded contradictory results.

"Right now theorists are having a lot of fun because we have two
possible outcomes, and maybe a third one, that both experiments are
incorrect and there's a third explanation," said Angela Olinto,
Associate Professor in Astronomy & Astrophysics at the University of
Chicago. 

A new experiment will likely pierce the secrets of these high-energy
cosmic rays and resolve the data conflict within a few years, said
Olinto, who will discuss the mystery in a session on cosmic-ray
astrophysics on Sunday, Feb. 16, at the American Association for the
Advancement of Science annual meeting in Denver.

High-energy cosmic rays are subatomic scraps of matter that fly
through the universe at nearly the speed of light. These particles hit 
Earth's atmosphere with the energy of a tennis ball traveling at 167 
miles an hour, Olinto said.

The Auger experiment is operated in Argentina by a collaboration of
more than 250 scientists in 16 nations. The project was initiated by
James Cronin, a Nobel laureate and University Professor Emeritus in
Physics at the University of Chicago, and Alan Watson of the
University of Leeds. Cronin and Watson named the project after Pierre
Auger, the scientist who discovered cosmic rays in 1938. Auger
conducted research at Chicago in 1942, launching hot-air balloon
experiments from the University's Stagg Field to study cosmic rays.

High-energy cosmic rays are relatively rare. They occur only once a
century over a given square kilometer patch of land -- thus the large
size of high-energy cosmic ray experiments. When complete, the Auger
experiment will consist of a grid of electronic instruments that
covers 3,000 square kilometers, an area more than half the size of the 
state of Delaware. 

When a cosmic ray strikes Earth, it reacts with atoms in the
atmosphere to create a cascade of a billion particles that shower the
ground. Utah's existing High Resolution Fly's Eye detects cosmic rays
by observing the fluorescent light they cause when they strike the
atmosphere. Japan's current Akeno Giant Air Shower Array detects the
cascade of secondary particles when they strike the ground. Auger will 
use both techniques on a larger scale and should resolve the data 
discrepancy between HiRes and AGASA. 

For years it seemed that cosmic rays emanated from all over the sky,
but that is no indication of where they originally came from. That is
because the rays are electrically charged and magnetic fields deflect
their paths as they travel through the universe. But at the very
highest energies, the rays will travel a direct line from their source 
to Earth, enabling scientists to pinpoint their origin.

AGASA has begun to see a hint of clusters of particles coming from the 
same region. AGASA also sees cosmic rays at unexpectedly high
energies. 

"That is really exciting, because if AGASA is really finding particles 
above the energy where we thought they wouldn't be, then there's a 
whole new class of cosmic particle accelerators that nobody has 
predicted," Olinto said.

HiRes, meanwhile, has detected neither clustering nor cosmic rays at
unexpectedly high energies. But Auger should be able to settle the
discrepancy as it grows from its current 30 detectors to 1,600
detectors in the coming years. "I believe AGASA has a better case for
being correct," Olinto said. "I also hope AGASA is correct -- it will
be a lot more fun." 

Some scientists have proposed that the high-energy cosmic rays are
produced by jets of matter emitted by supermassive black holes or by
gamma-ray bursts, which are the most powerful explosions in the
universe. Another possibility is topological defects, stresses and
strains comparable to faults and folds in the Earth's crust that
periodically release tremendous energies generated early in the
history of the universe. Spinning neutron stars within the Milky Way
galaxy are yet another explanation, according to a proposal put
forward by Olinto, Pasquale Blasi of the Astrophysical Observatory in
Arcetri, Italy, and Richard Epstein of Los Alamos National Laboratory.

"A very young neutron star could be spinning so fast, 3,000 times a
second, that its strong magnetic fields could hit these particles,
almost like a baseball bat, to incredible energies," Olinto said.

Only time will tell which theory, or another one entirely, proves
correct. 

"Right now we have a lot of fun things to debate," she said.

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