This letter reports the discovery of a remarkably hard spectrum source , HESS J1641 - 463 , by the High Energy Stereoscopic System ( H.E.S.S . )
in the very high energy ( VHE ) domain .
HESS J1641 - 463 remained unnoticed by the usual analysis techniques due to confusion with the bright nearby source HESS J1640 - 465 .
It emerged at a significance level of 8.5 standard deviations after restricting the analysis to events with energies above 4 TeV .
It shows a moderate flux level of \phi ( E > 1 TeV ) = ( 3.64 \pm 0.44 _ { stat } \pm 0.73 _ { sys } ) \times 10 ^ { -13 % } cm ^ { -2 } s ^ { -1 } , corresponding to 1.8 % of the Crab Nebula flux above the same energy , and a hard spectrum with a photon index of \Gamma = 2.07 \pm 0.11 _ { stat } \pm 0.20 _ { sys } .
It is a point-like source , although an extension up to a Gaussian width of \sigma = 3 arcmin can not be discounted due to uncertainties in the H.E.S.S .
point-spread function .
The VHE \gamma -ray flux of HESS J1641 - 463 is found to be constant over the observed period when checking time binnings from the year-by-year to the 28 minute exposure timescales .
HESS J1641 - 463 is positionally coincident with the radio supernova remnant SNR G338.5+0.1 .
No X-ray candidate stands out as a clear association ; however , Chandra and XMM- Newton data reveal some potential weak counterparts .
Various VHE \gamma -ray production scenarios are discussed .
If the emission from HESS J1641 - 463 is produced by cosmic ray protons colliding with the ambient gas , then their spectrum must extend close to 1 PeV .
This object may represent a source population contributing significantly to the galactic cosmic ray flux around the knee .