Watching the Ice. Locations of reflected cosmic ray events (red points) within ANITA's flight path around Antarctica (blue line). ANITA (inset) prepares to launch at the Columbia Scientific Balloon Facility near McMurdo Station, Antarctica.

David Saltzberg/ANITA Collaboration

Tuning In to Highest Energy Cosmic Rays

Nobody knows where in space the most powerful cosmic rays come from or how they acquire such mind bogglingly large energies—as much as a 3-centimeter-wide hail stone plummeting to Earth. But now physicists may have a new method for tracing these cosmic bullets. A team of 45 researchers working with a balloon-borne radio detector has spotted radio waves produced by ultrahigh-energy cosmic rays as the waves bounce off Antarctic ice.

Discovered nearly 100 years ago, cosmic rays are charged particles zinging in from the heavens. Many are thought to emerge from stellar explosions, yet the rays' origins remain murky, because interstellar magnetic fields deflect the electrically charged particles and make their paths too complex to follow. The highest energy rays might pack enough punch to fly straight from their sources, however. Such ultrahigh-energy cosmic rays can be detected in a few ways. When such a ray strikes the atmosphere, for example, it triggers an avalanche of charged particles that physicists can spot with particle detectors spread on the ground, enabling them to estimate the direction and energy of the original ray.

Because the highest energy rays arrive so rarely—at a rate of one per kilometer of Earth's surface per century—the arrays used to find them are huge. The Pierre Auger Observatory covers 3000 square kilometers of prairie in Argentina and comprises 1600 particle detectors and four batteries of telescopes. The Telescope Array in Utah has 576 particle detectors spread over 730 square kilometers and three telescope stations.

Now, researchers working with the Antarctic Impulsive Transient Antenna (ANITA) experiment say it may be possible to spot the rays with a much smaller detector, albeit one searching a much larger area. Consisting of a radio interferometer that hangs from a balloon floating at an altitude of 36 kilometers, ANITA flies over a million square kilometers of reflective ice in Antarctica. ANITA aims to detect other particles from space, the uncharged and elusive neutrinos, as they ricochet off the ice Those neutrinos should set off their own ice-bound particle avalanches, which will emit 1-nanosecond bursts of vertically polarized radio waves. The instrument also scans horizontally polarized waves as a control to check that it is properly screening out noise.

But when ANITA team members analyzed the signal and control data, they got a shock. The signal data showed one event, which is what the researchers expected, says David Saltzberg, a team member from the University of California, Los Angeles. "We looked in the control, and by surprise, we found six events. A deeper analysis finished a year later found there had actually been 16 events total."

The ANITA team reports in a paper in press at Physical Review Letters that those flashes in the control data were signals produced by ultrahigh-energy cosmic rays that had reflected off the ice. The researchers were able to make that cosmic ray connection because, for example, the flashes did not correlate with known human activity such as the walkie-talkie transmissions of other Antarctic scientists. By accident, ANITA has become a dual-purpose experiment, and the team is now tweaking their instrument to detect neutrinos and cosmic rays on its next flight in 2013.

"This is certainly an interesting and suggestive result," says Pierre Sokolsky, a physicist at the University of Utah in Salt Lake City and principal investigator of the Telescope Array project. However, Sokolsky says, it's not yet clear that ANITA can really compete with the arrays on the ground. For example, ANITA researchers' estimates of each ray's energy are about four times less precise than those from the Telescope Array or Auger, Sokolsky notes. "So I see this as an interesting technical development that still has a long way to go to prove itself as a reliable technique." Meanwhile, the controversy over where the rays come from continues.