They are almost certainly extragalactic. They cannot travel far to reach us. Accelerate low energy particles to high energy. One possible source is topological defects, the edges between different regions or domains of the big bang.
Cosmic rays are sub-atomic particles that have been accelerated up to close to the speed of light.
But supernovae cannot accelerate particles more than this, and there is no other known source of higher energy particles in the galaxy. These are the important questions that OWL hopes to answer.