We present the AGILE gamma-ray observations in the energy range 50 MeV - 10 GeV of the supernova remnant ( SNR ) W44 , one of the most interesting systems for studying cosmic-ray production .
W44 is an intermediate-age SNR ( \sim 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 our gamma-ray analysis to energies substantially lower than previous measurements which could not conclusively establish the nature of the radiation .
We find that gamma-ray emission matches remarkably well both the position and shape of the inner SNR shocked plasma .
Furthermore , the gamma-ray spectrum shows a prominent peak near 1 GeV with a clear decrement at energies below a few hundreds of MeV as expected from neutral pion decay .
Here we demonstrate that : ( 1 ) hadron-dominated models are consistent with all W44 multiwavelength constraints derived from radio , optical , X-ray , and gamma-ray observations ; ( 2 ) ad hoc lepton-dominated models fail to explain simultaneously the well-constrained gamma-ray and radio spectra , and require a circumstellar density much larger than the value derived from observations ; ( 3 ) the hadron energy spectrum is well described by a power-law ( with index s = 3.0 \pm 0.1 ) and a low-energy cut-off at E _ { c } = 6 \pm 1 GeV .
Direct evidence for pion emission is then established in an SNR for the first time .