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

Abstract

Context. 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. Aims. We study the broadband emission of these secondary pairs in the stellar photon and magnetic fields. Methods. Under certain assumptions regarding 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. Results. 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, also becoming softer at higher energies due to synchrotron loss dominance. Conclusions. 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 dominant over the primary gamma ray emission, and possibly detectable by the next generation of instruments.