Low-frequency radio spectrum of LS 5039 during periastron and apastron passages

We have recently studied LS 5039, a gamma-ray binary, with the Giant Metrewave Radio Telescope during its periastron and apastron passage. The results presented here show that the spectra are inverted at low frequency and the flux densities do not differ significantly for two different orbital phases. Assuming that the free-free absorption of radio in the stellar wind environment is responsible for the optically thick radio emission, we calculated the free-free absorption optical depth and constrained the height of the radio-emitting region from the orbital plane. The height is found to be around 1.6 au for a spherical stellar wind geometry. This estimate may change if the stellar wind is focused or the radio absorption is due to synchrotron self-absorption.