Studying the properties of the radio emitter in LS?5039

Context. LS?5039 is an X-ray binary that presents non-thermal radio emission. The radiation at ~5?GHz is quite steady and optically thin, consisting of a dominant core plus an extended jet-like structure. There is a spectral turnover around 1?GHz, and evidence of variability on timescales of 1?yr at 234?MHz.Aims. We investigate the radio emitter properties using the available broadband radio data, and assuming two possible scenarios to explain the turnover: free-free absorption in the stellar wind, or synchrotron self-absorption.Methods. We use the relationships between the turnover frequency, the stellar wind density, the emitter location, size and magnetic field, and the Lorentz factor of the emitting electrons, as well as a reasonable assumption regarding the energy budget, to infer the properties of the low-frequency radio emitter. Also, we put this information in context with the broadband radio data.Results. The location and size of the low-frequency radio emitter can be restricted to ?few?AU from the primary star, its magnetic field to ~3?? 10-3?1?G, and the electron Lorentz factors to ~10-100. The observed variability of the extended structures seen with VLBA would point to electron bulk velocities ?3?? 108?cm?s-1, whereas much less variable radiation at 5?GHz would indicate velocities for the VLBA core ?108?cm?s-1. The emission at 234?MHz in the high state would mostly come from a region larger than the dominant broadband radio emitter.Conclusions. We suggest a scenario in which secondary pairs, created via gamma-ray absorption and moving in the stellar wind, are behind the steady broadband radio core, whereas the resolved jet-like radio emission would come from a collimated, faster, outflow.