Fermilarge Area Telescope Detection of a Break in the Gamma-Ray Spectrum of the Supernova Remnant Cassiopeia A

We report on observations of the supernova remnant Cassiopeia A in the energy range from 100 MeV to 100 GeV using 44 months of observations from the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope . We perform a detailed spectral analysis of this source and report on a low-energy break in the spectrum at GeV. By comparing the results with models for the gamma-ray emission, we find that hadronic emission is preferred for the GeV energy range.