Non-Thermal Radio Emission from Colliding-Wind Binaries

TL;DR

This paper models the synchrotron radio emission in colliding-wind binaries, explaining observed variability and predicting detectability based on stellar wind absorption regions, with implications for future surveys.

Contribution

It introduces a detailed model of relativistic electron generation and radiative transfer in colliding-wind binaries, explaining observed radio variability.

Findings

The synchrotron emission region extends beyond the free-free absorption zone.

The model explains orbit-locked radio variability in Cyg OB2 No. 8A.

Application potential for future radio surveys like COBRaS.

Abstract

In colliding-wind binaries, shocks accelerate a fraction of the electrons up to relativistic speeds. These electrons then emit synchrotron radiation at radio wavelengths. Whether or not we detect this radiation depends on the size of the free-free absorption region in the stellar winds of both components. One expects long-period binaries to be detectable, but not the short-period ones. It was therefore surprising to find that Cyg OB2 No. 8A (P = 21.9 d) does show variability locked with orbital phase. To investigate this, we developed a model for the relativistic electron generation (including cooling and advection) and the radiative transfer of the synchrotron emission through the stellar wind. Using this model, we show that the synchrotron emitting region in Cyg OB2 No. 8A does extend far enough beyond the free-free absorption region to generate orbit-locked variability in the radio flux. This model can also be applied to other non-thermal emitters and will prove useful in interpreting observations from future surveys, such as COBRaS - the Cyg OB2 Radio Survey.