The orbital variability of gamma-ray emission in γ^2 Velorum

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 γ^2 Velorum (WR 11), composed of a Wolf–Rayet (WR) and an O star, has recently been associated with giga-electronvolt gamma-ray emission observed by Fermi -LAT, including showing 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 γ^2 Velorum using a physically motivated phase-dependent hadronic model. We considered the injection of accelerated relativistic protons based on the WR wind's kinetic energy intercepted at the wind collision region (WCR), and calculated the resulting phase-dependent hadronic gamma-ray emission assuming a proton conversion efficiency, η_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 was attenuated by γ--γ absorption, calculated via full angular integration over both stellar photon fields. Our model, including the attenuation resulting from the γ--γ absorption, successfully reproduces the observed SED and is consistent with the apastron-to-periastron flux ratio, resulting in a dip in emission at periastron passage, while an increase occurs during the apastron. Our findings support the conclusion that the observed orbital modulation is primarily driven by geometric variations in the WCR. This underscores the significant influence of evolving orbital geometry on the high-energy gamma-ray light curves of γ^2 Velorum. As CWBs emerge as a potential new class of high-energy gamma-ray sources, advancing our understanding will require more detailed magnetohydrodynamic modeling of the wind interaction dynamics in these systems.