On the formation of TeV radiation in LS 5039

TL;DR

This paper models the formation of TeV gamma-ray emission in LS 5039, analyzing how relativistic electrons accelerate and produce radiation, and how these processes depend on system geometry and physical conditions.

Contribution

It provides detailed numerical modeling of TeV gamma-ray production in LS 5039, considering acceleration efficiency, system inclination, and anisotropic inverse Compton scattering.

Findings

Gamma-ray emission is strongly orbital phase dependent.

The accelerator likely resides outside the binary system unless acceleration is highly efficient.

The model can fit observed spectra for specific orbital phases.

Abstract

The recent detections of TeV gamma-rays from compact binary systems show that relativistic outflows (jets or winds) are sites of effective acceleration of particles up to multi-TeV energies. In this paper, we discuss the conditions of acceleration and radiation of ultra-relativistic electrons in LS 5039, the gamma-ray emitting binary system for which the highest quality TeV data are available. Assuming that the gamma-ray emitter is a jet-like structure, we performed detailed numerical calculations of the energy spectrum and lightcurves accounting for the acceleration efficiency, the location of the accelerator, the speed of the emitting flow, the inclination angle of the system, as well as specific features related to anisotropic inverse Compton scattering and pair production. We conclude that the accelerator should not be deep inside the binary system unless we assume a very efficient acceleration rate. We show that within the IC scenario both the gamma-ray spectrum and flux are strongly orbital phase dependent. Formally, our model can reproduce, for specific sets of parameter values, the energy spectrum of gamma-rays reported by HESS for wide orbital phase intervals. However, the physical properties of the source can be constrained only by observations capable of providing detailed energy spectra for narrow orbital phase intervals ($\Delta\phi\ll 0.1$).