2016A&A...589A..51S

The supernova remnant (SNR) G338.3-0.0 spatially correlates with HESS J1640-465, which is considered the most luminous γ-ray source associated with a SNR in our Galaxy. The X-ray pulsar PSR J1640-4631 has been recently discovered within the SNR shell, which could favor a leptonic origin for the detected very-high-energy (VHE) emission. In spite of this, the origin of the VHE radiation from HESS J1640-465 has not been unambiguously clarified so far. Indeed, a hadronic explanation cannot be ruled out by current observations. On the basis of atomic (HI) and molecular (¹²CO) archival data, we determine, for the first time, the total ambient density of protons in the region of the G338.3-0.0/HESS J1640-465 system, a critical parameter for understanding the emission mechanisms at very high energies. The value obtained is in the 100-130cm^–3 range. Besides this, we developed a new hadronic model to describe the spectral energy distribution (SED) of the HESS J1640-465 source, which includes the latest total γ-ray cross-section for proton-proton collisions available in the literature. By using the assessed ambient proton density, we found that the total energy in accelerated protons required to fit the data is 5.4^+4.7_–2.3x10⁴⁹erg and 1.6^+1.4_–0.7x10⁵⁰erg for a source distance of 8.5 and 13kpc, respectively. The case where the source distance is 8.5kpc agrees with the typical scenario in which the energy released is on the order of 10⁵¹erg and ∼10% of that energy is transferred to the accelerated protons, whereas the case corresponding to a source distance of 13kpc requires either a higher value of the energy released in the explosion or a larger energy fraction to accelerate protons.