  Novae are thermonuclear explosions on a white dwarf surface fueled by mass
accreted from a companion star. Current physical models posit that shocked
expanding gas from the nova shell can produce X-ray emission but emission at
higher energies has not been widely expected. Here, we report the Fermi Large
Area Telescope detection of variable gamma-ray (0.1-10 GeV) emission from the
recently-detected optical nova of the symbiotic star V407 Cygni. We propose
that the material of the nova shell interacts with the dense ambient medium of
the red giant primary, and that particles can be accelerated effectively to
produce pi0 decay gamma-rays from proton-proton interactions. Emission
involving inverse Compton scattering of the red giant radiation is also
considered and is not ruled out.
