Simulating three-dimensional nonthermal high-energy photon emission in colliding-wind binaries

TL;DR

This paper presents a comprehensive 3D simulation of high-energy photon emission in colliding-wind binary systems, accounting for anisotropic inverse Compton scattering, bremsstrahlung, pion decay, and photon opacity effects, revealing emission variations with orbital parameters.

Contribution

It introduces a detailed 3D numerical model combining hydrodynamics and particle transport to simulate nonthermal high-energy emission in colliding-wind binaries, including anisotropic effects and orbital orientation dependencies.

Findings

Transition from hadron- to lepton-dominated emission with increasing stellar separation

Spectral energy distribution varies significantly with orbital orientation

Photon flux and spectral features depend on binary system geometry

Abstract

Massive stars in binary systems have long been regarded as potential sources of high-energy gamma rays.The emission is principally thought to arise in the region where the stellar winds collide and accelerate relativistic particles which ubsequently emit gamma rays. On the basis of a three-dimensional distribution function of high-energy particles in the wind collision region - as obtained by a numerical hydrodynamics and particle transport model - we present the computation of the three-dimensional nonthermal photon emission for a given line of sight. Anisotropic inverse Compton emission is modelled using the target radiation field of both stars. Photons from relativistic bremsstrahlung and neutral pion decay are computed on the basis of local wind plasma densities. We also consider photon photon opacity effects due to the dense radiation fields of the stars. Results are shown for different stellar separations of a given binary system comprising of a B star and a Wolf-Rayet star. The influence of orbital orientation with respect to the line of sight is also studied by using different orbital viewing angles. For the chosen electron-proton injection ratio of 0.01, we present the ensuing photon emission in terms of two-dimensional projections maps, spectral energy distributions and integrated photon flux values in various energy bands. Here, we find a transition from hadron-dominated to lepton-dominated high-energy emission with increasing stellar separations. In addition, we confirm findings from previous analytic modeling that the spectral energy distribution varies significantly with orbital orientation.