On the orbital variability of gamma-ray emission in $γ^2$ Velorum

Abstract: Colliding wind binaries (CWBs) are promising sources of high-energy gamma-ray emission driven by shock acceleration of particles at wind interaction zones. The nearby CWB system $\gamma^2$ Velorum (WR 11), composed of a Wolf-Rayet (WR) and an O-star, has been recently associated with GeV gamma-ray emission observed by Fermi-LAT, including evidence of orbital variability. This offers a valuable opportunity to test models of phase-dependent hadronic emission and absorption in CWBs. We aim to explain both the spectral energy distribution (SED) and orbital variability of gamma-ray emission from $\gamma^2$ Velorum using a physically motivated phase-dependent hadronic model. We consider the injection of accelerated relativistic protons based on the WR wind's kinetic energy intercepted at the wind collision region (WCR), and calculate the resulting phase-dependent hadronic gamma-ray emission assuming a proton conversion efficiency $\eta_p$ and accounting for energy-dependent diffusion, advection, conical shock interception and the evolution of the effective acceleration volume, assumed to scale with the WCR, with the orbital phase. Gamma-ray emission from hadronic interactions is attenuated by $\gamma$ - $\gamma$ absorption, calculated via full angular integration over both stellar photon fields. Our model successfully reproduces the observed SED and is consistent with the apastron-to-periastron flux ratio, resulting in a dip in emission at periastron passage and an increase during apastron. Our findings support the conclusion that the observed orbital modulation is primarily driven by geometric variations of the WCR. This underscores the significant influence of evolving orbital geometry on the high-energy gamma-ray light curves of $\gamma^2$ Velorum.