Scientists have finally located a source of the most energetic rays. Starting with a single signal—a flash of light in a detector at the South Pole—and combining it with telescope data from a collaboration of over a thousand people, astrophysicists have traced the origin of some of Earth’s cosmic rays to a blazar, a type of galaxy, 4 billion light years away. “We’ve learned that these active galaxies are responsible for accelerating particles and cosmic rays,” says physicist Francis Halzen of the University of Wisconsin-Madison.
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