Evidence for Cosmic Ray Acceleration in Cassiopeia A

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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.