Origin of high-energy cosmic rays more mysterious, again

New data weaken link between rare, energetic particles and giant black holes

DENVER — New data cast doubt on the origin of high-energy cosmic rays, which previous evidence indicated came mostly from galaxies that house supermassive black holes.

An investigation of more of the rare, energetic events suggests that fewer than half may come from this type of galaxy, David Thomas of Colorado State University in Fort Collins reported May 3 at a meeting of the American Physical Society.

High-energy cosmic rays are charged particles, such as protons, that smash into Earth’s atmosphere packing as much punch as a fast-pitch baseball. Researchers for years have puzzled over the origin of these rare, unusually energetic particles.

Two years ago, astronomers working at the world’s biggest cosmic ray observatory, the 3,000-square-kilometer Pierre Auger Observatory in Malargüe, Argentina, reported that these energetic cosmic rays come from regions of the sky coinciding with the location of galaxies known to house giant black holes. Among 27 cosmic rays with energies greater than 57 billion billion electronvolts, 20 of them could be traced back to such locations.

The finding appeared to pinpoint the origin of high-energy cosmic rays. The work also suggested how the charged particles may have gotten their extra energy: They might have been revved up by jets of high-speed gas hurled into space by supermassive black holes as material spiraled into these gravitational beasts (SN: 11/10/07, p. 291). But with double the amount of data on high-energy cosmic rays now collected by the observatory, the correlation is no longer so clear-cut, Thomas says. Only 40 percent of the energetic cosmic rays can be traced back to galaxies housing supermassive black holes, compared to more than 70 percent with the smaller sample, he says.

Thomas says he and his colleagues will have to collect about 10 times more of these rare, energetic cosmic ray events to determine whether or not the particles originate from the environs of giant black holes. That gives an even greater impetus, he says, for plans now underway to build a sister site to Pierre Auger in southeast Colorado. The Colorado facility would have seven times the collecting area of the Argentinean site, he says.

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