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 0.1GeV to 10TeV. We used 65 hours of VERITAS data to cover 200 GeV - 10 TeV, and 10.8 years of $\mathit{\text{Fermi}}$-LAT data to cover 0.1-500 GeV. The spectral analysis of $\mathit{\text{Fermi}}$-LAT data shows a significant spectral curvature around $1.3\pm {0.4}_{stat}$ GeV that is consistent with the expected spectrum from pion decay. Above this energy, the joint spectrum from $\mathit{\text{Fermi}}$-LAT and VERITAS deviates significantly from a simple power-law, and is best described by a power-law with spectral index of $2.17\pm {0.02}_{stat}$ with a cut-off energy of $2.3\pm {0.5}_{stat}$ 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 multi-wavelength spectrum, a minimum magnetic field inside the remnant of $B_{\mathrm{m}\mathrm{i}\mathrm{n}}\approx 150\mu \mathrm{G}$ is deduced.