Gamma-ray binaries : pulsars in disguise?

Context. LS 5039 and LS I+61°303 are unique amongst high-mass X-ray binaries (HMXB) for their spatially-resolved radio emission and their counterpart at >GeV gamma-ray energies, canonically attributed to non-thermal particles in an accretion-powered relativistic jet. The only other HMXB known to emit very high-energy (VHE) gamma-rays, PSR B1259-63, harbours a non-accreting millisecond pulsar. Aims. The purpose is to investigate whether the interaction of the relativistic wind from a young pulsar with the wind from its stellar companion, as in PSR B1259-63, constitutes a viable scenario for explaining the observations of LS 5039 and LS I+61°303. Emission arises from the shocked pulsar wind material, which then flows away to large distances in a comet-shape tail, reproducing on a smaller scale what is observed in isolated, high motion pulsars interacting with the interstellar medium. Methods. The timescales for acceleration and radiation of particles at the shock between the pulsar wind and stellar wind are calculated. Simple expectations for the spectral energy distribution (SED) are derived and are shown to depend on very few input parameters. Detailed modelling of the particle evolution is attempted and compared to the observations from radio to TeV energies. Results. Acceleration at the shock provides high-energy electrons that steadily emit synchrotron in X-rays and inverse Compton scatter stellar light to γ -rays. Electrons streaming out of the system emit at IR frequencies and below. The overall aspect of the SEDs is adequately reproduced for standard values of the parameters. The morphology of the radio tail can mimic a microquasar jet. Good agreement is found with the published VLBI map of LS 5039 and predictions are made on the expected change in appearance with orbital phase. Conclusions. The pulsar wind scenario provides a common, viable framework for interpreting the emission from all three γ -ray binaries.