Unraveling the high and very-high energy emission components of LS 5039

Abstract

Since the detection of gamma-ray binaries in the GeV range with Fermi, the GeV and TeV spectra of these systems have challenged our picture of their high-energy emission. The exponential cutoff at a few GeV, combined with the hard spectrum at TeV and the anticorrelation between the two bands, points towards two different origins for the GeV and TeV emission from LS 5039 and LS I +61 303. If these systems are powered by a young pulsar, one of the locations could be the apex of the contact discontinuity between the pulsar and stellar winds, located between the pulsar and the star. This location is a good candidate for emission of the GeV component, but the strong photon field density from the star makes it an unsuitable TeV emitter owing to pair production opacity. However, the rotation of the pulsar around the star gives rise to a bending of the wind interaction region by Coriolis forces at distances larger than the size of the binary system. We show that the particles accelerated in the pulsar wind shock at this location could be responsible for the TeV component in gamma-ray binaries. The spectral and lightcurve resulting from this simple two-emitter model match satisfactorily the Fermi and HESS observations of LS 5039, and provide a starting point for a future broadband analysis, from radio to TeV, of the properties of the system.