A Serendipitous Discovery of GeV Gamma-Ray Emission from Supernova 2004dj in a Survey of Nearby Star-forming Galaxies with Fermi-LAT

The interaction between a supernova ejecta and the circumstellar medium drives a strong shock wave that accelerates particles (i.e., electrons and protons). The radio and X-ray emission observed after the supernova explosion can be interpreted as synchrotron emission from accelerated electrons. The accelerated protons are expected to produce GeV–TeV gamma-ray emission via proton–proton collisions, but the flux is usually low since only a small fraction of the supernova kinetic energy is converted into the shock energy at the very early time. The low gamma-ray flux of the nearest supernova explosion, SN 1987A, agrees with this picture. Here we report a serendipitous discovery of a fading GeV gamma-ray source in spatial coincidence with one of the nearest and brightest supernova—SN 2004dj from our gamma-ray survey of nearby star-forming galaxies with Fermi-LAT. The total gamma-ray energy released by SN 2004dj is about 6 × 1047 erg. We interpret this gamma-ray emission arising from the supernova ejecta interacting with a surrounding high-density shell, which decelerates the ejecta and converts ∼1% of the SN kinetic energy to relativistic protons.