The nonthermal supernova remnant (SNR) RX J1713.7-3946 has recently been sh
own to be a site of cosmic-ray (CR) electron acceleration to TeV energies.
Here we present evidence that this remnant is also accelerating CR nuclei.
Such energetic nuclei can interact with ambient interstellar gas to produce
high-energy gamma rays via the decay of neutral pions. We associate the un
identified EGRET GeV gamma-ray source, 3EG J1714-3857, with a very massive
(similar to3 x 10(5) M-.) and dense (similar to 500 nucleons cm(-3)) molecu
lar cloud interacting with SNR RX J1713.7-3946. Direct evidence for such in
teraction is provided by observations of the lowest two rotational transiti
ons of CO molecules in the cloud; as in other clear cases of interaction, t
he CO (J = 2 --> 1)/CO (J = 1 --> 0) ratio is significantly enhanced. Since
the cloud is of low radio and X-ray brightness, electrons cannot be respon
sible for the bulk of the GeV emission there. A picture thus emerges in whi
ch both electrons and nuclei are being accelerated by the SNR: whereas the
relativistic electrons dominate the local nonthermal radio, X-ray, and TeV
emission, the shock-accelerated CR protons and ions (hadrons) are exposed t
hrough their interactions in the adjacent massive cloud, leading to the obs
erved GeV emission via the gamma decay of neutral pions.