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

Abstract

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 ${\hat{I}}^3$-ray binary 1FGL J1018.6-5856 have resulted in a source detection significance of more than 9$\ddot{I}ƒ$, and the detection of variability ($\ddot{I}{‡}^2$/$\hat{I}½$ of 238.3/155) in the emitted ${\hat{I}}^3$-ray flux. This variability confirms the association of HESS J1018-589 A with the high-energy ${\hat{I}}^3$-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 $\hat{I}“=2.20\pm 0.14_{\mathrm{stat}}\pm 0.2_{\mathrm{sys}}$. Emission is detected up to ~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 ~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 3$\ddot{I}ƒ$. The shape of the VHE phaseogram and measured spectrum suggest a low inclination, low eccentricity system with a modest impact from VHE ${\hat{I}}^3$-ray absorption due to pair production ($\ddot{I}„$ < 1 at 300 GeV).

Accepted for publication in A&A