Secondary emission behind the radio outflows in gamma-ray binaries?

TL;DR

This paper investigates how secondary electron-positron pairs produced by gamma-ray absorption in gamma-ray binaries contribute to radio emission, affecting spectrum, variability, and morphology interpretations.

Contribution

It demonstrates that secondary pairs significantly influence radio observations, highlighting the importance of accounting for their emission in gamma-ray binary studies.

Findings

Secondary pairs produce a hard radio spectrum.

Radio emission shows orbital motion of the peak.

Extended structures follow spiral-like trajectories.

Abstract

Several binary systems consisting of a massive star and a compact object have been detected above 100 GeV in the Galaxy. In most of these sources, gamma-rays show a modulation associated to the orbital motion, which means that the emitter should not be too far from the bright primary star. This implies that gamma-ray absorption will be non negligible, and large amounts of secondary electron-positron pairs will be created in the stellar surroundings. In this work, we show that the radio emission from these pairs should be accounted for when interpreting the radio spectrum, variability, and morphology found in gamma-ray binaries. Relevant features of the secondary radio emission are the relatively hard spectrum, the orbital motion of the radio peak center, and the extended radio structure following a spiral-like trajectory. The impact of the stellar wind free-free absorption should not be neglected.