phase-I observations of the plane of the Milky Way HESS J1741 − 302: a hidden accelerator in the Galactic plane

The H.E.S.S. Collaboration has discovered a new very high energy (VHE, E > 0 . 1 TeV) γ -ray source, HESSJ1741 − 302, located in the Galactic plane. Despite several attempts to constrain its nature, no plausible counterpart has been found so far at X-ray and MeV / GeV γ -ray energies, and the source remains unidentified. An analysis of 145-h of observations of HESSJ1741 − 302 at VHEs has revealed a steady and relatively weak TeV source ( ∼ 1% of the Crab Nebula flux), with a spectral index of Γ = 2 . 3 ± 0 . 2 stat ± 0 . 2 sys , extending to energies up to 10 TeV without any clear signature of a cut-o ff . In a hadronic scenario, such a spectrum implies an object with particle acceleration up to energies of several hundred TeV. Contrary to most H.E.S.S. unidentified sources, the angular size of HESSJ1741 − 302 is compatible with the H.E.S.S. point spread function at VHEs, with an extension constrained to be below 0.068 ◦ at a 99% confidence level. The γ -ray emission detected by H.E.S.S. can be explained both within a hadronic scenario, due to collisions of protons with energies of hundreds of TeV with dense molecular clouds, and in a leptonic scenario, as a relic pulsar wind nebula, possibly powered by the middle-aged (20 kyr) pulsar PSRB1737 − 30. A binary scenario, related to the compact radio source 1LC358.266 + 0.038 found to be spatially coincident with the best fit position of HESSJ1741 − 302, is also envisaged.