Non-thermal emission from secondary pairs in close TeV binary systems

TL;DR

This paper models the broadband emission from secondary electron-positron pairs created by gamma-ray absorption near hot stars, predicting detectable radio to gamma-ray signals and emphasizing the importance of magnetic fields in shaping the emission.

Contribution

It provides a detailed calculation of the steady-state energy distribution and emission spectra of secondary pairs in close TeV binary systems, highlighting their potential observability.

Findings

Secondary pairs produce detectable radio to gamma-ray emission.

Synchrotron emission peaks around X-ray energies.

Inverse Compton emission peaks near 10 GeV.

Abstract

Massive hot stars produce dense ultraviolet (UV) photon fields in their surroundings. If a very high-energy (VHE) gamma-ray emitter is located close to the star, then gamma-rays are absorbed in the stellar photon field, creating secondary (electron-positron) pairs. We study the broadband emission of these secondary pairs in the stellar photon and magnetic fields. Under certain assumptions on the stellar wind and the magnetic field in the surroundings of a massive hot star, we calculate the steady state energy distribution of secondary pairs created in the system and its radiation from radio to gamma-rays. Under the ambient magnetic field, possibly high enough to suppress electromagnetic (EM) cascading, the energy of secondary pairs is radiated via synchrotron and single IC scattering producing radio-to-gamma-ray radiation. The synchrotron spectral energy distribution (SED) is hard, peaks around X-ray energies, and becomes softer. The IC SED is hard as well and peaks around 10 GeV, becoming also softer at higher energies due to synchrotron loss dominance. The radio emission from secondary pairs is moderate and detectable as a point-like and/or extended source. In X-rays, the secondary pair synchrotron component may be dominant. At energies <10 GeV, the secondary pair IC radiation may be the dominant primary gamma-ray emission and possibly detectable by the next generation of instruments.