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The interaction between a supernova ejecta and the circum-stellar mediumdrives a strong shock wave which accelerates particles (i.e., electrons andprotons). The radio and X-ray emission observed after the supernova explosionconstitutes the evidence of the electron acceleration. The accelerated protonsare expected to produce GeV-TeV gamma-ray emission via $pp$ collisions, but theflux is usually low since only a small fraction of the supernova kinetic energyis converted into the shock energy at the very early time. The low gamma-rayflux of the nearest supernova explosion, SN 1987A, agrees with this picture.Here we report a serendipitous discovery of a fading GeV gamma-ray source inspatial coincidence with the second nearest supernova--SN 2004dj from ourgamma-ray survey of nearby star-forming galaxies with Fermi-LAT. The totalgamma-ray energy released by SN 2004dj is about $6\times10^{47}{\rm erg}$. Weinterpret this gamma-ray emission arising from the supernova ejecta interactingwith a surrounding high-density shell, which decelerates the ejecta andconverts ~1% of the ejecta's kinetic energy to relativistic protons. Inaddition, our gamma-ray survey of nearby star-forming galaxies discovers GeVemissions from two star-forming galaxies, i.e., Arp 299 and M33, for the firsttime.

A serendipitous discovery of GeV gamma-ray emission from supernova 2004dj in a survey of nearby star-forming galaxies with Fermi-LAT