  Combining archival data taken at radio and infrared wavelengths with
state-of-the-art measurements at X-ray and gamma-ray energies, we assembled a
broadband spectral energy distribution (SED) of Cas A, a young supernova
remnant. Except for strong thermal emission at infrared and X-ray wavelengths,
the SED is dominated by non-thermal radiation. We attempted to model the
non-thermal SED with a two-zone leptonic model which assumes that the radio
emission is produced by electrons that are uniformly distributed throughout the
remnant while the non-thermal X-ray emission by electrons that are localized in
regions near the forward shock. Synchrotron emission from the electrons can
account for data from radio to X-ray wavelengths. Much of the GeV-TeV emission
can also be explained by a combination of bremsstrahlung emission and
inverse-Compton scattering (mainly of infrared thermal photons). However, the
model cannot fit a distinct feature at GeV energies. This feature can be well
accounted for by adding a pion-zero emission component to the model, providing
evidence for cosmic ray production in Cas A. We discuss the implications of the
results.
