Constraints on cosmic-ray origin from TeV gamma-ray observations of supernova remnants

If supernova remnants (SNRs) are the site of cosmicray acceleration, the associated nuclear interactions should result in an observable flux of γ-rays for the nearest SNRs. Measurements of the TeV γ-ray flux from six nearby, radio-bright SNRs have been made with the Whipple Observatory imaging air Čerenkov telescope over the period September 1993 to June 1996. No significant emission has been detected and upper limits on the >300 GeV flux are reported. Three of these SNRs (IC443, γ-Cygni and W44) are spatially coincident with low-latitude otherwise unidentified sources detected with the Energetic Gamma-Ray Experiment Telescope (EGRET). If the EGRET γ-ray fluxes result from cosmic-ray interactions then the EGRETandWhipple data are found to be collectively inconsistent with a cosmic-ray source spectrum flatter than∼ E−2.4. The Whipple upper limits for IC443 and γ-Cygni are also inconsistent with a priori predictions if these remnants are indeed expanding into regions where the average density of the interstellarmedium is enhanced by the presence ofmolecular clouds. These data weaken the case for the simplest models of shock acceleration and energy dependent propagation of cosmic rays.