Particle acceleration and nonthermal radio emission in binaries of early-type stars

It is shown that the stellar wind collisions in early-type binaries may be strong sources of both high-energy particles and nonthermal emission. Nonthermal radio emission may be observed only if the orbital period of the binary is more than some critical value. For a typical Wolf-Rayet binary the value of the critical period is of the order of a month when the frequency of observation is a few GHz. Observations of nonthermal radio emission of the remarkable Wolf-Rayet binary WR 140, such as luminosity, spectrum, time variability in the process of orbital motion, the size of nonthermal radio source, etc., are explained by a model in which its nonthermal radio emission is generated at the site of stellar wind collision. Considerations of shock acceleration theory suggest that the WR 140 system may be a strong source of γ-rays. It is proposed that EGRET could both measure the time dependence of the γ-ray emission as well as resolve an inverse Compton spectrum from a pion decay spectrum and thereby provide information on the electron versus proton content of the relativistic particles.