Radio, X-ray, and gamma-ray Emission Models of the Colliding Winds Binary WR 140

Abstract

We use hydrodynamical models of the wind-collision region (WCR) in the archetype colliding-wind system WR140 to determine the spatial and spectral distribution of the radio, X-ray and gamma-ray emission from shock accelerated electrons. Our calculations are for orbital phase 0.837 when the observed radio emission is close to maximum. Using the observed thermal X-ray emission together with the radio emission to constrain the mass-loss rates, we find that the O-star mass-loss rate is consistent with recent reductions for O4-5 supergiants. We demonstrate that radio VLBI observations of the WCR fail to constrain the opening angle. The observed low frequency turnover at ~3 GHz in the radio emission is due to free-free absorption, since models based on the Razin effect have an unacceptably large fraction of energy in non-thermal particles. The index of the non-thermal electron energy distribution is flatter than the canonical value for diffusive shock acceleration, namely p<2. Several mechanisms are discussed that could lead to such an index. Tighter constraints on p and the nature of the shocks in WR140 will be obtained from future observations at MeV and GeV energies, for which we generally predict lower fluxes than previous work. Since the high stellar photon fluxes prevent the acceleration of electrons beyond gamma > 1e5-1e6, TeV emission from CWB systems will provide unambiguous evidence of pion-decay emission from accelerated ions. We finish by commenting on the emission and physics of the multiple wind collisions in dense stellar clusters, paying particular attention to the Galactic Centre (abridged).