Research Note

Studying the properties of the radio emitter in LS 5039

Max Planck Institut für Kernphysik, Saupfercheckweg 1, Heidelberg 69117, Germany e-mail: vbosch@mpi-hd.mpg.de

Received: 16 September 2008
Accepted: 4 November 2008

Abstract

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 ~. 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.