Evidence for proton acceleration up to TeV energies based on VERITAS and Fermi-LAT observations of the Cas A SNR

We present a study of gamma-ray emission from the core-collapse supernova remnant Cas A in the energy range from 0.1 GeV to 10 TeV. We used 65 hr of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) data to cover 200 GeV-10 TeV, and 10.8 yr of Fermi-Large Area Telescope (LAT) data to cover 0.1-500 GeV. The spectral analysis of Fermi-LAT data shows a significant spectral curvature around 1.3 0.4(stat) GeV that is consistent with the expected spectrum from pion decay. Above this energy, the joint spectrum from Fermi-LAT and VERITAS deviates significantly from a simple power law, and it is best described by a power law with a spectral index of 2.17 0.02(stat) and a cutoff energy of 2.3 0.5(stat) TeV. These results, along with radio, X-ray, and gamma-ray data, are interpreted in the context of leptonic and hadronic models. Assuming a one-zone model, we exclude a purely leptonic scenario and conclude that proton acceleration up to at least 6 TeV is required to explain the observed gamma-ray spectrum. From modeling of the entire multiwavelength spectrum, a minimum magnetic field inside the remnant of B-min 150 mu G is deduced.