Egret observations of radio-bright supernova remnants

The gamma-ray emission associated with radio-bright supernova remnants has been studied using data from the EGRET instrument aboard the Compton Gamma Ray Observatory. Five sources from The Second EGRET Catalog of High-Energy Gamma-Ray Sources are shown to be coincident with supernova remnants. The two strongest pointlike gamma-ray excesses which are coincident with supernova remnants are 2EG J2020+4026 with gamma Cyg and 2EG J0618+2234 with IC 443. The gamma-ray flux, E > 100 MeV, from the region of these two supernova remnants is (12.6 +/- 0.7) x 10(-7) photons cm(-2) s-1 and (5.0 +/- 0.4) x 10-7 photons cm(-2) s(-1), respectively. The differential gamma-ray flux of both supernova remnants is consistent with a single power law, F proportional to E(-alpha), with alpha = 2.07 +/- 0.05 and 1.97 +/- 0.07 for gamma Cyg and IC 443, respectively. A cosmic-ray origin model based on shock acceleration of cosmic rays is consistent with our results suggesting supernova remnants as an acceleration site, and possibly the origin, of Galactic cosmic rays. Other mechanisms to explain the possible gamma-ray emission from supernova remnants are also discussed in the context of the EGRET observations.