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The discovery of bright gamma-ray emission coincident with supernova remnant(SNR) W51C is reported using the Large Area Telescope (LAT) on board the FermiGamma-ray Space Telescope. W51C is a middle-aged remnant (~10^4 yr) withintense radio synchrotron emission in its shell and known to be interactingwith a molecular cloud. The gamma-ray emission is spatially extended, broadlyconsistent with the radio and X-ray extent of SNR W51C. The energy spectrum inthe 0.2-50 GeV band exhibits steepening toward high energies. The luminosity isgreater than 1x10^{36} erg/s given the distance constraint of D>5.5 kpc, whichmakes this object one of the most luminous gamma-ray sources in our Galaxy. Theobserved gamma-rays can be explained reasonably by a combination of efficientacceleration of nuclear cosmic rays at supernova shocks and shock-cloudinteractions. The decay of neutral pi-mesons produced in hadronic collisionsprovides a plausible explanation for the gamma-ray emission. The product of theaverage gas density and the total energy content of the accelerated protonsamounts to 5x10^{51}(D/6kpc)^2 erg/cm^3. Electron density constraints from theradio and X-ray bands render it difficult to explain the LAT signal as due toinverse Compton scattering. The Fermi LAT source coincident with SNR W51C shedsnew light on the origin of Galactic cosmic rays.

Fermi-LAT Discovery of Extended Gamma-ray Emission in the Direction of Supernova Remnant W51C