NEWS OF THE WEEKASTROPHYSICS

40025 JANUARY 2008

VOL 319SCIENCE

Published by AAAS

Because it’s bigger, Auger will see more ofthe rare rays above 60 EeV. So Sokolsky plans
to focus on lower energies and especially on akink in the spectrum of rays near 4 EeV thatmight mark the point at which rays fromwithin our galaxy peter out and those frombeyond take over. The team has proposed a“low-energy extension” of 100 more-tightlyspaced detectors and two more telescope
stations to measure showers with between
0.03 EeV and 10 EeV. “For this, the Telescope
Array and especially the low-energy extension
is an excellent instrument,” Gran Sasso’s
Berezinsky says. Auger should have similar
additions in place in 2009.
In contrast, Fukushima hopes
to pursue the highest energy rays.
Many physicists now doubt theexcess reported by AGASA, as
neither HiRes nor Auger has
seen it (Science, 13 July 2007,p. 178). Still, Fukushima and his
Japanese colleagues hope toprobe the discrepancy between
AGASA and HiRes.
Telescope Array will also measure a ray’s energy more precisely
than Auger can, Fukushima says.
Auger comprises fourtelescope batteries
and nearly 1500 particle detectors. But
Auger’s detectors
are of tanks ofwater, which produces light called
Cherenkov radiation when a particle zips throughit at near-light
speed. Telescope Array’s detectors
are sheets of plastic scintillator that emit
light through another mechanism. “Definitely we are measuring the cosmic rays in adifferent way and with better energy resolution,” Fukushima says.
Ultimately, Auger and Telescope Array
may be forced to work together. The Telescope Array team hopes someday to expand
its observatory, and the Auger team plans tobuild a far-bigger array in Colorado in a fewyears. The two arrays could end up beingcombined, Sokolsky says. “Whatever wescientists might think about it, that’s going
to be imposed on us by the funding agencies,” he predicts. For now, however, thecompetition is on.
–ADRIAN CHO

www.sciencemag.org

Downloaded from www.sciencemag.org on January 27, 2008

In a field in which bigger is usually better, excess: They saw only two such rays. The
what can you hope to achieve with a new HiRes and AGASA groups studied the rays
experiment that’s only a quarter as large as its using different techniques, however. So towell-established rival? Plenty, say 117 physi- resolve the discrepancy, they eventuallycists mainly from Japan and the United States decided to build an array that would use both.
who have just started taking data with a cosWhen a cosmic ray strikes the atmosphere,
mic ray observatory that covers 730 square it sets off an avalanche of particles called ankilometers of western Utah.
extensive air shower. AGASA sampled theDubbed Telescope Array, the observa- shower using 111 particle detectors spread
tory aims to spot the most energetic sub- over 100 square kilometers. The shower also
atomic particles from space. Such ultra- causes the air to fluoresce, and HiRes studied
high-energy cosmic rays pack as much that light using twin batteries of telescopes.
energy as a golf ball hitting a fairway, and Telescope Array comprises 503 particle
they strike Earth at a rate of 1 per century detectors and 38 telescopes in three batteries.
per square kilometer. Interest inthem grew 10 years ago, whenJapanese physicists reported anodd excess of the highest energy
rays. It surged last year, when thegargantuan Pierre Auger Observatory in Argentina traced therays to certain galaxies (Science,9 November 2007, p. 896).
Telescope Array aims to test
the Auger result and to decipher
the nature of the rays. It enters thefray as an underdog: Although it’s
bigger than the city of Chicago,
it’s only a quarter the size ofAuger, which has been taking
data since 2004. But team members say Telescope Array has key
technological advantages, andothers say it may be better for pursuing certain questions. “This is avery important experiment,” says
Veniamin Berezinsky, a theoristat Gran Sasso National Labora- On the range. Spaced 1.2 kilometers
tory in Assergi, Italy.
apart, Telescope Array’s particle detectors
The Telescope Array collabo- stretch across the scrub. Its telescopes
ration formed when two rival (inset) perch on nearby hilltops.
groups merged. Physicists measure the energies of cosmic rays in exa–electronJapan put up $13 million for the $16 million
volts, and in 1998, researchers with the Akeno array, but researchers never considered conGiant Air Shower Array (AGASA) near structing it there. “Building a fluorescenceTokyo reported seven rays with energies detector in Japan is impossible,” says Masaki
above 100 EeV. By 2002, they saw 11. That Fukushima of the University of Tokyo.
was about 10 times more than expected; if the “Because of the humidity, the transparency ofrays were protons, then on average, inter- the air is very limited.” The project got its
actions with the cosmic microwave background inapposite name because the Japanese hadshould have sapped their energy to 60 EeV previously proposed an array of 10 telescopes
before they had traveled 200 light-years.
with no particle detectors. “Once you propose
Some theorists took the excess as evidence something you don’t change the name,that the rays were born in decays of exotic par- because no one will know what you’re talking
ticles lingering nearby. But physicists with the about,” says Pierre Sokolsky of the University
High Resolution Fly’s Eye (HiRes) detector in of Utah, Salt Lake City. “So even though it
Dugway, Utah, argued that there was no makes no sense, the name stuck.”

CREDITS: MASAKI FUKUSHIMA AND SHIGEHARU UDO (INSET)/INSTITUTE FOR COSMIC RAY RESEARCH, UNIVERSITY OF TOKYO“Little” Cosmic Ray Observatory Aims to Make a Big Mark