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

Abstract

Re-observations with the HESS telescope array of the very high-energy (VHE) source HESS J1018–589A that is 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 J1018–589A with the high-energy γ-ray binary detected by Fermi-LAT and also confirms the point-like source as a new VHE binary system. The spectrum of HESS J1018–589A 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 differential flux level is (2:9 ± 0:4) x 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 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 Nσ > 3σ. The shape of the VHE phaseogram and measured spectrum suggest a low-inclination, low-eccentricity system with a modest impact from VHE Υ-ray absorption due to pair production (ζ≲1 at 300 GeV).