A pair of astronomers may have solved a long-standing puzzle about the source of ultrahigh energy cosmic rays, particles that slam into the atmosphere with 100 million times the energies reached in the largest particle accelerators. They have traced a handful of these particles back to highly energetic active galactic nuclei, the turbulent centers of distant galaxies that may harbor massive black holes. The finding, reported in the 26 October issue of Physical Review Letters, could upset current notions about the nature of ultrahigh energy cosmic rays.
Astrophysicists have figured that the highest energy cosmic rays would have to originate in the neighborhood of our galaxy. That's because any charged particle, like a proton, traveling farther would be slowed to lower energy levels by the microwave background--the low-energy radiation that pervades the universe. But no one has been able to find a nearby source for the ultrahigh energy rays.
Glennys Farrar, now at New York University, and Peter Biermann of the Max Planck Institute for Radioastronomy in Bonn suspected a more distant source: a highly energetic class of active galactic nuclei that have intense magnetic fields, which might be capable of accelerating particles to high energies. Because each incoming cosmic ray sets off a chain reaction in the atmosphere that ends in a detectable shower of electrons or positrons, Farrar and Biermann could figure out the approach angle of five cosmic rays. In each case, the path of the incoming ray could be traced back to a previously identified active galactic nucleus. The probability of the cosmic rays lining up with such galaxies by pure chance is only 0.5%.
"If the correlation is as good as they claim, then it's very, very suggestive that we may well have found the source of these extremely high-energy cosmic rays," says Raymond Protheroe, an astrophysicist at University of Adelaide in Australia. But he adds that this would upset the current assumption that cosmic rays are made up of protons or atomic nuclei, because they could never retain such high energies over such long distances. If Farrar and Biermann are right, "whatever's getting to us cannot possibly be a proton," says Protheroe.
Farrar and Biermann hypothesize that the ultrahigh energy cosmic ray particles could be new neutral particles or neutrinos, which would not interact with the microwave background. But given that the analysis rests on just a handful of events, they say, much more work will be needed to close the case.