Modulation of 1FGL J1018.6-5856 VHE -ray light-curves due to [ípsilon]-[ípsilon] absorption under different orbital parameters

Abstract

Binary systems composed of a neutron star or a black hole, and emitting in very high energy (VHE) -rays, have attracted a lot of interest in the last decade. One of these systems, 1FGL J1018.6-5856, is known to be composed of massive star of spectral type O6V((f)) and has an orbital period of 16.58 days, from the periodic emission in both X-rays and radio. Apart from this, the orbital parameters of the system are not well known. Since the VHE -rays interact with the photons from the massive companion, mainly via [ípsilon]-[ípsilon] absorption, the emitted VHE light-curve depends on the orbital configuration. We created a Fortran code to compute the [ípsilon]-[ípsilon] absorption and simulate the emitted light-curve, assuming an intrinsic constant VHE ux, with different orbital parameters. Comparing our simulated results with H.E.S.S observations of 1FGL J1018.6-5856 we are able to fit the best possible parameters within the observation uncertainties, obtaining an eccentricity e = 0:35 +- 0:05, an inclination i = 42º +- 5º and a periastron angle w = 83º +- 10º. The results are coherent with the range of values suggested in the literature. However, these results are not in agreement if the GeV emission is only produced by anisotropic Inverse Compton scattering