Monte-Carlo Simulations of Radio Emitting Secondaries in Gamma-Ray Binaries

TL;DR

This paper uses Monte-Carlo simulations to study the properties and morphology of radio emission from secondary pairs in gamma-ray binaries, highlighting their potential contribution to observed radio structures.

Contribution

It introduces a detailed Monte-Carlo model to analyze the spatial, spectral, and variability characteristics of secondary radio emission in gamma-ray binaries, including specific application to LS 5039.

Findings

Secondary radio emission appears as an extended structure of a few milliarcseconds.

The emission can have fluxes up to approximately 10 mJy.

Modulation of radio emission depends on system parameters like luminosity, eccentricity, and wind ionization.

Abstract

Several binary systems that contain a massive star have been detected in both the radio band and at very high energies. In the dense stellar photon field of these sources, gamma-ray absorption and pair creation are expected to occur, and the radiation from these pairs may contribute significantly to the observed radio emission. We aim at going deeper in the study of the properties, and in particular the morphology, of the pair radio emission in gamma-ray binaries. We apply a Monte-Carlo code that computes the creation location, the spatial trajectory and the energy evolution of the pairs produced in the binary system and its surroundings. The radio emission produced by these pairs, with its spectral, variability and spatial characteristics, is calculated as it would be seen from a certain direction. A generic case is studied first, and then the specific case of LS 5039 is also considered. We find that, confirming previous results, the secondary radio emission should appear as an extended radio structure of a few milliarcseconds size. This radiation would be relatively hard, with fluxes up to ~ 10 mJy. Modulation is expected depending on the gamma-ray production luminosity, system eccentricity, and wind ionization fraction, and to a lesser extent on the magnetic field structure. In gamma-ray binaries in general, the pairs created due to photon-photon interactions can contribute significantly to the core, and generate an extended structure. In the case of LS 5039, the secondary radio emission is likely to be a significant fraction of the detected core flux, with a marginal extension.