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HESS J1641-463 is a unique source discovered by the High Energy StereoscopicSystem (H.E.S.S.) telescope array in the multi-TeV domain. The source had beenpreviously hidden in the extended tail of emission from the bright nearbysource HESS J1640-465. However, the analysis of the very-high-energy (VHE) datafrom the region at energies above 4 TeV revealed this new source at asignificance level of 8.5$\sigma$. HESS J1641-463 showed a moderate flux levelF(E > 1 TeV) = (3.64 +/- 0.44_stat +/- 0.73_sys) 10^-13 cm^-2s^-1,corresponding to 1.8% of the Crab Nebula flux above the same energy, and a hardspectrum with a photon index Gamma = 2.07 +/- 0.1_stat +/- 0.20_sys. The lightcurve was investigated for evidence of variability, but none was found on bothshort (28-min observation) and long (yearly) timescales. HESS J1641-463 ispositionally coincident with the radio supernova remnant (SNR) G338.5+0.1.There is no clear X-ray counterpart of the SNR, although Chandra and XMM-Newtondata reveal some weak emission that may be associated. If the emission fromHESS J1641-463 is produced by cosmic ray protons colliding with the ambientgas, then the proton spectrum extends up to 0.1 PeV (99% confidence level) andlikely to higher energies, > 0.27 PeV (90% confidence level). If this is thecase, then HESS J1641-463 may be a member of a larger source populationcontributing to the Galactic cosmic-ray flux around the knee.

HESS J1641-463, a very hard spectrum TeV gamma-ray source in the Galactic plane