Studying the properties of the radio emitter in LS 5039

Abstract

LS 5039 is an X-ray binary that presents non-thermal radio emission. The radiation at $\sim 5$ GHz is quite steady and optically thin, consisting on a dominant core plus an extended jet-like structure. There is a spectral turnover around 1 GHz, and evidence of variability at timescales of 1 yr at 234 MHz. 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. 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 on 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. The location and size of the low-frequency radio emitter can be restricted to $\ga$ few AU from the primary star, its magnetic field to $\sim 3\times 10^{-3}-1$ G, and the electron Lorentz factors to $\sim 10-100$. The observed variability of the extended structures seen with VLBA would point to electron bulk velocities $\ga 3\times 10^8$ cm s$^{-1}$, whereas much less variable radiation at 5 GHz would indicate velocities for the VLBA core $\la 10^8$ 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. 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.