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Free keywords:
Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
MPINP:
HESS - Abteilung Hofmann
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 $\gamma$-ray binary
1FGL J1018.6-5856 have resulted in a source detection significance of more than
9$\sigma$, and the detection of variability ($\chi^2$/$\nu$ 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 very-high-energy
binary system. The spectrum of HESS J1018-589 A is best fit with a power-law
function with photon index $\Gamma = 2.20 \pm 0.14_{\rm stat} \pm 0.2_{\rm
sys}$. Emission is detected up to ~20 TeV. The mean differential flux level is
$(2.9 \pm 0.4)\times10^{-13}$ 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$\sigma$. The shape of the VHE phaseogram and measured spectrum suggest a low
inclination, low eccentricity system with a modest impact from VHE $\gamma$-ray
absorption due to pair production ($\tau$ < 1 at 300 GeV).
