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We present an analysis of the gamma-ray data obtained with the Large AreaTelescope (LAT) onboard the Fermi Gamma-ray Space Telescope in the direction ofSNR W49B (G43.3-0.2). A bright unresolved gamma-ray source detected at asignificance of 38 sigma is found to coincide with SNR W49B. The energyspectrum in the 0.2-200 GeV range gradually steepens toward high energies. Theluminosity is estimated to be 1.5x10^{36} (D/8 kpc)^2 erg s^-1 in this energyrange. There is no indication that the gamma-ray emission comes from a pulsar.Assuming that the SNR shell is the site of gamma-ray production, the observedspectrum can be explained either by the decay of neutral pi mesons producedthrough the proton-proton collisions or by electron bremsstrahlung. Thecalculated energy density of relativistic particles responsible for the LATflux is estimated to be remarkably large, U_{e,p}>10^4 eV cm^-3, for eithergamma-ray production mechanism.

Fermi-LAT Study of Gamma-ray Emission in the Direction of Supernova Remnant W49B