Title: MHD Models of Gamma-ray Emission in WR 11,
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.