Abstract
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.

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