Discovery of variable VHE γ - ray emission from the binary system 1 FGL J 1018 . 6 – 5856

Re-observations with the H.E.S.S. telescope array of the very-high-energy (VHE) source HESS J1018–589 A coincident with the Fermi-LAT γ-ray binary 1FGL J1018.6–5856 have resulted in a source detection significance of more than 9σ, and the detection of variability (χ2/ν of 238.3/155) in the emitted γ-ray flux. This variability confirms the association of HESS J1018–589 A with the high-energy γ-ray binary detected by Fermi-LAT, and also confirms the point-like source as a new very-high-energy binary system. The spectrum of HESS J1018–589 A is best fit with a power-law function with photon index Γ = 2.20 ± 0.14stat ± 0.2sys. Emission is detected up to ∼20 TeV. The mean differential flux level is (2.9 ± 0.4) × 10−13 TeV−1cm−2s−1 at 1 TeV, equivalent to ∼1% of the flux from the Crab Nebula at the same energy. Variability is clearly detected in the night-by-night lightcurve. When folded on the orbital period of 16.58 days, the rebinned lightcurve peaks in phase with the observed X-ray and high-energy phaseograms. The fit of the H.E.S.S. phaseogram to a constant flux provides evidence of periodicity at the level of Nσ > 3σ. The shape of the VHE phaseogram and measured spectrum suggest a low inclination, low eccentricity system with a modest impact from VHE γ-ray absorption due to pair production (τ . 1 at 300 GeV).