The middle-aged supernova remnant ( SNR ) W44 has recently attracted attention because of its relevance regarding the origin of Galactic cosmic-rays .
The gamma-ray missions AGILE and Fermi have established , for the first time for a SNR , the spectral continuum below 200 MeV which can be attributed to neutral pion emission .
Confirming the hadronic origin of the gamma-ray emission near 100 MeV is then of the greatest importance .
Our paper is focused on a global re-assessment of all available data and models of particle acceleration in W44 , with the goal of determining on a firm ground the hadronic and leptonic contributions to the overall spectrum .
We also present new gamma-ray and CO NANTEN2 data on W44 , and compare them with recently published AGILE and Fermi data .
Our analysis strengthens previous studies and observations of the W44 complex environment and provides new information for a more detailed modeling .
In particular , we determine that the average gas density of the regions emitting 100 MeV- 10 GeV gamma-rays is relatively high ( n \sim 250 - 300 cm ^ { -3 } ) .
The hadronic interpretation of the gamma-ray spectrum of W44 is viable , and supported by strong evidence .
It implies a relatively large value for the average magnetic field ( B \geq 10 ^ { 2 } \mu G ) in the SNR surroundings , sign of field amplification by shock-driven turbulence .
Our new analysis establishes that the spectral index of the proton energy distribution function is p _ { 1 } = 2.2 \pm 0.1 at low energies and p _ { 2 } = 3.2 \pm 0.1 at high energies .
We critically discuss hadronic versus leptonic-only models of emission taking into account simultaneously radio and gamma-ray data .
We find that the leptonic models are disfavored by the combination of radio and gamma-ray data .
Having determined the hadronic nature of the gamma-ray emission on firm ground , a number of theoretical challenges remains to be addressed .