The mystery of the origin of the strongest cosmic rays has deepened as new clues into key suspects, the most powerful explosions in the universe, suggest they are likely not potential culprits, researchers say. Cosmic rays are charged subatomic particles that streak to Earth from deep in outer space. A few rare cosmic rays are extraordinarily powerful, with energies up to 100 million times greater than any attained by human-made particle colliders, such as CERN’s Large Hadron Collider.

Study co-author Francis Halzen, principal investigator at the IceCube Neutrino Observatory, said the source of these cosmic rays is unknown but there are two current ideas in the scientific community: Supermassive black holes at the centers of active galaxies or gamma-ray bursts resulting from the collapse of stars to a black hole. New evidence may now rule out gamma-ray bursts as sources of these ultra-high-energy cosmic rays.