Fermi-Lat Study of Gamma-Ray Emission in the Direction of Supernova Remnant W49b

We present an analysis of the gamma-ray data obtained with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope in the direction of SNR W49B (G43.3−0.2). A bright unresolved gamma-ray source detected at a significance of 38σ is found to coincide with SNR W49B. The energy spectrum in the 0.2–200 GeV range gradually steepens toward high energies. The luminosity is estimated to be 1.5 × 10 36 ( D /8 kpc) 2 erg s −1 in this energy range. There is no indication that the gamma-ray emission comes from a pulsar. Assuming that the supernova remnant (SNR) shell is the site of gamma-ray production, the observed spectrum can be explained either by the decay of neutral π mesons produced through the proton–proton collisions or by electron bremsstrahlung. The calculated energy density of relativistic particles responsible for the LAT flux is estimated to be remarkably large, U e , p >10 4 eV cm −3 , for either gamma-ray production mechanism.