Implications of the VHE γ-Ray Spectral Variability of LS 5039

Evidence for orbital modulation of the very high energy (VHE) γ-ray emission from the high-mass X-ray binary and microquasar LS 5039 has recently been reported by the HESS collaboration. The observed flux modulation was found to go in tandem with a change in the GeV – TeV spectral shape, which may partially be a result of γγ absorption in the intense radiation field of the massive companion star. In a recent paper [1], I have presented a parameter study of the γγ absorption effects in this system. For a range of plausible locations of the VHE γ-ray emission region and the allowable range of viewing angles, the de-absorbed, intrinsic VHE γ-ray spectra and total VHE photon fluxes and luminosities were calculated and compared to luminosity constraints based on Bondi-Hoyle limited wind accretion onto the compact object in LS 5039. It was found that (1) it is impossible to choose the viewing angle and location of the VHE emission region in a way that the intrinsic spectra in superior and inferior conjunction are identical; (2) if the VHE luminosity is limited by wind accretion from the companion star and the system is viewed at an inclination angle of i > 40o, the emission is most likely beamed by a larger Doppler factor than inferred from the dynamics of the large-scale radio outflows; (3) the still poorly constrained viewing angle between the line of sight and the jet axis is most likely substantially smaller than the maximum of ∼ 64o inferred from the lack of eclipses. (4) Consequently, the compact object is more likely to be a black hole rather than a neutron star. (5) There is a limited range of allowed configurations for which the expected VHE neutrino flux would actually anti-correlate with the observed VHE γ-ray emission. If hadronic models for the γ-ray production in LS 5039 apply, a solid detection of the expected VHE neutrino flux and its orbital modulation with km3 scale water-Cherenkov neutrino detectors might require the accumulation of data over more than 3 years.