Physical properties of the gamma-ray binary LS 5039 through low and high frequency radio observations

TL;DR

This study analyzes the radio spectrum of gamma-ray binary LS 5039 across 0.15-15 GHz, revealing a persistent, slightly variable emission with a spectral turnover at 0.5 GHz, supporting a pulsar wind scenario with specific physical parameters.

Contribution

It provides the first evidence of the Razin effect in a gamma-ray binary and constrains the physical properties of LS 5039's emission region using multi-frequency radio observations.

Findings

Radio emission is persistent with less than 25% variability.

Spectral turnover at ~0.5 GHz explained by synchrotron self-absorption and Razin effect.

Derived physical parameters include magnetic field ~20 mG and electron density ~4x10^5 cm^-3.

Abstract

We have studied in detail the 0.15-15 GHz radio spectrum of the gamma-ray binary LS 5039 to look for a possible turnover and absorption mechanisms at low frequencies, and to constrain the physical properties of its emission. We have analysed two archival VLA monitorings, all the available archival GMRT data and a coordinated quasi-simultaneous observational campaign conducted in 2013 with GMRT and WSRT. The data show that the radio emission of LS 5039 is persistent on day, week and year timescales, with a variability $\lesssim 25~\%$ at all frequencies, and no signature of orbital modulation. The obtained spectra reveal a power-law shape with a curvature below 5 GHz and a turnover at $\sim0.5$ GHz, which can be reproduced by a one-zone model with synchrotron self-absorption plus Razin effect. We obtain a coherent picture for a size of the emitting region of $\sim0.85~\mathrm{mas}$, setting a magnetic field of $B\sim20~\mathrm{mG}$, an electron density of $n_{\rm e}\sim4\times10^5~{\rm cm^{-3}}$ and a mass-loss rate of $\dot M\sim5\times10^{-8}~{\rm M_{\odot} yr^{-1}}$. These values imply a significant mixing of the stellar wind with the relativistic plasma outflow from the compact companion. At particular epochs the Razin effect is negligible, implying changes in the injection and the electron density or magnetic field. The Razin effect is reported for first time in a gamma-ray binary, giving further support to the young non-accreting pulsar scenario.