Neutral pion emission from accelerated protons in the Supernova Remnant W44

GIULIANI A., CARDILLO M., TAVANI M., FUKUI Y., YOSHIIKE S., TORII, DUBNER G., CASTELLETTI G., BARBIELLINI G., BULGARELLI A., CARAVEO P., COSTA E. Y COL.

The Astrophysical Journal Letters

IOP Science

Lugar: Philadelphia; Año: 2011 vol. 742 p. 1 - 5

2041-8205

We present the AGILE gamma-ray observations in the energy range 50 MeV-10 GeV of the Supernova Remnant W44, one of the most interesting systems for studying 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 (MCs), 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 +/-  0.1) and a low-energy cut-off at Ec=6 +/- 1 GeV. Direct evidence for pion emission is then established in a SNR for the first time