  The interaction between a supernova ejecta and the circum-stellar medium
drives a strong shock wave which accelerates particles (i.e., electrons and
protons). The radio and X-ray emission observed after the supernova explosion
constitutes the evidence of the electron acceleration. The accelerated protons
are expected to produce GeV-TeV gamma-ray emission via $pp$ 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 the second nearest 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 $6\times10^{47}{\rm erg}$. 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 ejecta's kinetic energy to relativistic protons. In
addition, our gamma-ray survey of nearby star-forming galaxies discovers GeV
emissions from two star-forming galaxies, i.e., Arp 299 and M33, for the first
time.
