Supernova observations solve the mystery of cosmic-ray origins

Supernova remnant IC 443, seen in X-ray light. Proton collisions in this remnant produced pions, which decayed to gamma rays - which allowed astronomers to solve the mystery of cosmic ray origins.

NASA/DOE/Fermi LAT Collaboration, Chandra X-ray Observatory

On October 15, 1991, a high-energy proton from deep space struck Earth's upper atmosphere. Known as the "Oh My God Particle", this proton was by far the highest energy cosmic ray ever seen. This one proton's energy was equivalent to a regulation soccer ball traveling at 15 meters per second (34 miles per hour). In the two decades following, observers spotted several similarly energetic cosmic rays, which left a big question: what was accelerating these protons to higher speeds than anything we can achieve in on Earth?

The most likely candidates are supernova explosions, some of the most violent events in the Universe. Until now, however, no one could establish a clear association between cosmic rays and the remains of a supernova. Now a new study used the Fermi Large Area Telescope to make the connection. M. Ackermann and colleagues established the source of at least some of the highest energy cosmic rays beyond reasonable doubt.