MHD models of gamma-ray emission in WR 11

Abstract

Recent reports claiming tentative association of the massive star binary system γ2 Velorum (WR 11) with a high-energy γ-ray source observed by Fermi-LAT contrast the so-far exclusive role of η Carinae as the hitherto only detected γ-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 γ-ray emission – its radiation will unavoid-ably be of hadronic origin owing to the strong radiation fields in the binary system which inhibit the acceleration of electrons to energies sufficiently 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 γ-ray emission originates at a ∼400 R⊙ wide region at the apex.