Discovery of variable VHE gamma-ray emission from the binary system 1FGL J1018.6-5856

Re-observations with the HESS telescope array of the very high-energy (VHE) source HESS J1018–589 A that is coincident with the Fermi-LAT gamma-ray binary 1FGL J1018.6–5856 have resulted in a source detection significance of more than 9\sigma and the detection of variability ($\chi^{2}/v$ of 238.3/155) in the emitted gamma-ray flux. This variability confirms the association of HESS J1018–589 A with the high-energy $\gamma$-ray binary detected by Fermi-LAT and also confirms the point-like source as a new VHE binary system. The spectrum of HESS J1018–589 A is best fit with a power-law function with photon index $\Gamma = 2.20 \pm 0.14_{stat} \pm 0.2_{sys}$. Emission is detected up to $\sim$20 TeV. The mean differential flux level is $(2.9 \pm 0.4) \times 10^{-13} TeV^{-1} cm^{-2} s^{-1}$ at 1 TeV, equivalent to $\sim$1% of the flux from the Crab Nebula at the same energy. Variability is clearly detected in the night-by-night light curve. When folded on the orbital period of 16.58 days, the rebinned light curve peaks in phase with the observed X-ray and high-energy phaseograms. The fit of the HESS phaseogram to a constant flux provides evidence of periodicity at the level of $N\sigma>3\sigma$. The shape of the VHE phaseogram and measured spectrum suggest a low-inclination, low-eccentricity system with amodest impact from VHE gamma-ray absorption due to pair production ($\tau \leqslant $1 at 300 GeV).