We present a study of \gamma -ray emission from the core-collapse supernova remnant Cas A in the energy range from 0.1 \mathrm { GeV } to 10 \mathrm { TeV } .
We used 65 hours of VERITAS data to cover 200 GeV - 10 TeV , and 10.8 years of Fermi -LAT data to cover 0.1-500 GeV .
The spectral analysis of Fermi -LAT data shows a significant spectral curvature around 1.3 \pm 0.4 _ { stat } \mathrm { 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 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 } \mathrm { 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 { min } } \approx 150 \mathrm { \mu G } is deduced .