# MHD Models of Gamma-ray Emission in WR 11

## TL;DR

This paper models gamma-ray emission in the binary system WR 11 using 3D magneto-hydrodynamics, predicting hadronic origin of gamma rays and variability over the orbit, to evaluate its potential as a gamma-ray source.

## Contribution

It provides the first detailed 3D MHD model of WR 11's wind-collision region, predicting nonthermal emission and clarifying the gamma-ray emission mechanism.

## Key findings

- Gamma-ray emission from WR 11 is likely hadronic in origin.
- Orbital variability significantly affects gamma-ray flux.
- Emission mainly originates near the wind collision apex.

## Abstract

Recent reports claiming tentative association of the massive star binary system gamma^2 Velorum (WR 11) with a high-energy gamma-ray source observed by Fermi-LAT contrast the so-far exclusive role of Eta Carinae as the hitherto only detected gamma-ray emitter in the source class of particle-accelerating colliding-wind binary systems. We aim to shed light on this claim of association by providing dedicated model predictions for the nonthermal photon emission spectrum of WR 11. We use three-dimensional magneto-hydrodynamic modeling to trace the structure and conditions of the wind-collision region of WR 11 throughout its 78.5 day orbit, including the important effect of radiative braking in the stellar winds. A transport equation is then solved in the wind-collision region to determine the population of relativistic electrons and protons which are subsequently used to compute nonthermal photon emission components. We find that - if WR 11 be indeed confirmed as the responsible object for the observed gamma-ray emission - its radiation will unavoidably be of hadronic origin owing to the strong radiation fields in the binary system which inhibit the acceleration of electrons to energies suffciently high for observable inverse Compton radiation. Different conditions in wind-collision region near the apastron and periastron configuration lead to significant variability on orbital time scales. The bulk of the hadronic gamma-ray emission originates at a 400 solar radii wide region at the apex.