High-energy gamma-ray emission from supernova remnants ( SNRs ) has provided a unique perspective for studies of Galactic cosmic-ray acceleration .
Tycho ’ s SNR is a particularly good target because it is a young , type Ia SNR that is well-studied over a wide range of energies and located in a relatively clean environment .
Since the detection of gamma-ray emission from Tycho ’ s SNR by VERITAS and Fermi -LAT , there have been several theoretical models proposed to explain its broadband emission and high-energy morphology .
We report on an update to the gamma-ray measurements of Tycho ’ s SNR with 147 hours of VERITAS and 84 months of Fermi -LAT observations , which represents about a factor of two increase in exposure over previously published data .
About half of the VERITAS data benefited from a camera upgrade , which has made it possible to extend the TeV measurements toward lower energies .
The TeV spectral index measured by VERITAS is consistent with previous results , but the expanded energy range softens a straight power-law fit .
At energies higher than 400 GeV , the power-law index is 2.92 \pm 0.42 _ { \mathrm { stat } } \pm 0.20 _ { \mathrm { sys } } .
It is also softer than the spectral index in the GeV energy range , 2.14 \pm 0.09 _ { \mathrm { stat } } \pm 0.02 _ { \mathrm { sys } } , measured by this study using Fermi –LAT data .
The centroid position of the gamma-ray emission is coincident with the center of the remnant , as well as with the centroid measurement of Fermi –LAT above 1 GeV .
The results are consistent with an SNR shell origin of the emission , as many models assume .
The updated spectrum points to a lower maximum particle energy than has been suggested previously .