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We present the AGILE gamma-ray observations in the energy range 50 MeV - 10GeV of the supernova remnant (SNR) W44, one of the most interesting systems forstudying cosmic-ray production. W44 is an intermediate-age SNR (20, 000 years)and its ejecta expand in a dense medium as shown by a prominent radio shell,nearby molecular clouds, and bright [SII] emitting regions. We extend ourgamma-ray analysis to energies substantially lower than previous measurementswhich could not conclusively establish the nature of the radiation. We findthat gamma-ray emission matches remarkably well both the position and shape ofthe inner SNR shocked plasma. Furthermore, the gamma-ray spectrum shows aprominent peak near 1 GeV with a clear decrement at energies below a fewhundreds of MeV as expected from neutral pion decay. Here we demonstrate that:(1) hadron-dominated models are consistent with all W44 multiwavelengthconstraints derived from radio, optical, X-ray, and gamma-ray observations; (2)ad hoc lepton-dominated models fail to explain simultaneously thewell-constrained gamma-ray and radio spectra, and require a circumstellardensity much larger than the value derived from observations; (3) the hadronenergy spectrum is well described by a power-law (with index s = 3.0 \pm 0.1)and a low-energy cut-off at Ec = 6 \pm 1 GeV. Direct evidence for pion emissionis then established in an SNR for the first time.

Neutral pion emission from accelerated protons in the supernova remnant W44