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

TL;DR

This study investigates the low-frequency radio spectrum of the gamma-ray binary LS 5039 during different orbital phases, revealing spectral inversion and estimating the radio emitting region's height based on free-free absorption.

Contribution

It provides the first detailed analysis of LS 5039's low-frequency radio spectrum during periastron and apastron, estimating the radio emission region height from stellar wind absorption.

Findings

Spectral inversion at low frequencies observed.

No significant flux density variation between orbital phases.

Estimated radio emission region height is around 1.6 AU.

Abstract

We have recently studied LS 5039, a gamma-ray binary, with Giant Meterwave Radio Telescope (GMRT) during its periastron and apastron passage. The results presented here show that the spectra are inverted at the low frequency and the flux densities do not differ significantly for two different orbital phases. Assuming that the free-free absorption of radio in 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 focussed or the radio absorption is due to synchrotron self-absorption.