Non-thermal radio emission from O stars : binary versus single

For some of the brightest O stars, non-thermal radio emission is observed. It is believed that synchrotron radiation from relativistic electrons can account for this emission. In single stars, these electrons are thought to be accelerated by wind-embedded shocks. Taking into account that wind-embedded shocks weaken as they move out with the wind, non-thermal radio emission models produce a radio spectrum with a positive slope (as a function of frequency) as opposed to the observed negative slope. This result suggests that the non-thermal radio emission is not associated with wind-embedded shocks, but with a wind-colliding shock. This means that all non-thermal O stars should be binaries