Recently published results using seven years of Fermi-LAT data shed new light on the still puzzling source class of particle-accelerating colliding-wind binary (CWB) systems.
While the claimed association of the system $\gamma^2$ Velorum (WR 11) with a high-energy $\gamma$-ray source contrasts the exclusivity of $\eta$ Carinae as the hitherto only detected $\gamma$-ray emitter of that sort, the low upper limits obtained for WR 140 strengthen the question why this system with all its similarities to the $\gamma$-ray bright $\eta$ Carinae remains still unseen.
We use three-dimensional magneto-hydrodynamic modeling (MHD) to investigate the structure and conditions of the wind-collision region (WCR) in these three systems, including the important effect of radiative braking in the stellar winds. A transport equation is then solved throughout the computational domain to study the propagation of relativistic electrons and protons. The resulting distributions of particles are subsequently used to compute nonthermal photon emission components.
With the above procedure, we obtained first model results that can account for the weak detection of $\gamma^2$ Velorum, the strong detection of $\eta$ Carinae, and the non-detection of WR 140 in a similar computational setup.