Relativistic fluid modelling of gamma-ray binaries

Abstract

Context. In the first paper of this series, we presented a numerical model for the non-thermal emission of gamma-ray binaries in a pulsar-wind-driven scenario. Aims. We apply this model to one of the best-observed gamma-ray binaries, the LS 5039 system. Methods. The model involves a joint simulation of the interaction between the pulsar wind and the stellar wind and the transport of electron pairs from the pulsar wind accelerated at the emerging shock structure. We compute the synchrotron and inverse Compton emission in a post-processing step while consistently accounting for relativistic beaming and γγ-absorption in the stellar radiation field. Results. The wind interaction leads to the formation of an extended, asymmetric wind collision region that develops strong shocks, turbulent mixing, and secondary shocks in the turbulent flow. Both the structure of the collision region and the resulting particle distributions show significant orbital variation. In addition to the acceleration of particles at the bow-like pulsar wind and the Coriolis shock, the model naturally accounts for the re-acceleration of particles at secondary shocks that contribute to the emission at very-high-energy (VHE) gamma-rays. The model successfully reproduces the main spectral features of LS 5039. While the predicted light curves in the high-energy and VHE gamma-ray band are in good agreement with observations, our model still does not reproduce the X-ray to low-energy gamma-ray modulation, which we attribute to the employed magnetic field model. Conclusions. We successfully model the main spectral features of the observed multi-band, non-thermal emission of LS 5039 and thus further substantiate a wind-driven interpretation of gamma ray binaries. Open issues relate to the synchrotron modulation, which might be addressed through a magnetohydrodynamic extension of our model.