Discovery of variable VHE -ray emission from the binary system 1FGLJ1018.6-5856

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 -ray binary 1FGL J1018.6‐5856 have resulted in a source detection significance of more than 9 , and the detection of variability ( 2 / of 238.3/155) in the emitted -ray flux. This variability confirms the association of HESS J1018‐589 A with the high-energy -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 = 2:20 0:14stat 0:2sys. Emission is detected up to 20 TeV. The mean di erential flux level is (2:9 0:4) 10 13 TeV 1 cm 2 s 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 N > 3 . The shape of the VHE phaseogram and measured spectrum suggest a low inclination, low eccentricity system with a modest impact from VHE -ray absorption due to pair production ( . 1 at 300 GeV).