The supernova remnant IC 443, popularly known as the Jellyfish Nebula, is located 5,000 light-years away and is thought to be about 10,000 years old. It is the expanding shock wave and debris formed when a massive star exploded.
Scientists previously could not determine which atomic particles are responsible for emissions from the interstellar gas clouds, because cosmic ray protons and electrons give rise to gamma-rays with similar energies. After analyzing four years of data, Fermi scientists saw a distinguishable feature in the gamma-ray emission of IC 443. The feature is caused by a short-lived particle called a neutral pion, which is produced when cosmic-ray protons smash into normal protons. The pion quickly decays into a pair of gamma-rays, in an emission that exhibits a swift and characteristic decline at lower energies. The low-end cutoff acts as a fingerprint, providing clear proof that the culprits in IC 443 are protons.