For the first time, a probable source of ultra-high-energy cosmic rays (UHECRs) has been identified in the sky, says the astrophysicist who reported the discovery at the San Diego meeting of the American Astronomical Society in January. The researcher, Glennys Farrar of New York University, says the identification came following a statistical analysis of data from two cosmic-ray observatories, one in Japan and the other in Utah.
The observatories — Japan’s Akeno Giant Air Shower Array (AGASA) and the National Science Foundation’s High Resolution Fly’s Eye Air Fluorescence Detector (HiRes) in Utah — produced cosmic-ray data that differ in observational method and sensitivity. In addition, each observatory’s set of data, taken singly, was insufficient to indicate a source location. However, Farrar and her HiRes collaborators found a way to analyze both observatories’ records in combination.
Data cluster
They found a group of four ultra-high-energy cosmic-ray events coming from a single direction in the sky. Each event was detected either as a minute streak of light left in the atmosphere or a splat of particles detected on the ground. The chances of these four being coincidentally aligned, they say, are less than 1 in 100. With a source location tentatively pinpointed, Farrar returned to the data, checking lower energies — and found a fifth event detected by HiRes in the same area of sky.
These five UHECRs arrived over a 10-year span. Their locations fit within a circle about three Full Moons wide, centered on 11h17m right ascension and 57° north declination. This lies near the 2nd-magnitude star Merak (Beta Ursae Majoris) in the bowl of the Big Dipper.
The ultra-high-energy cosmic rays, Farrar says, have much higher energies than ordinary cosmic rays. They are mostly protons, but moving at nearly the speed of light. Each tiny bit of matter packs the kinetic energy of a fast-moving bowling ball. Ultra-high-energy cosmic rays are extremely rare, however, with one hitting a square kilometer (0.4 square mile) on Earth every century on average. Only about 100 such events have ever been detected in the Northern Hemisphere.
