Radio, X-ray, and γ-ray emission models of the colliding-wind binary WR 140

Abstract

We use hydrodynamical models of the wind-collision region in the archetype colliding-wind system WR 140 to determine the spatial and spectral distributions of the radio, X-ray, and γ-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 at this phase in conjunction with the radio emission to constrain the mass-loss rates, we find that the O star mass-loss rate is consistent with the reduced estimates for O4-5 supergiants by Fullerton, Massa & Prinja, and the wind-momentum ratio, η= 0.02. This is independent of the opening angle deduced from radio very long baseline interferometry observations of the WCR that we demonstrate fail to constrain the opening angle. We show that the 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 electrons. We find that the spectral index of the non-thermal electron energy distribution is flatter than the canonical value for diffusive shock acceleration, namely p 2 does not exclude the possibility of shock modification, but stronger evidence than that which currently exists is necessary for its support. Tighter constraints on p and the nature of the shocks in WR 140 will be obtained from future observations at MeV and GeV energies, for which we generally predict lower fluxes than those in previous works. Since the high stellar photon fluxes prevent the acceleration of electrons beyond γ≳ 105–106, TeV emission from colliding-wind binary 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.