Spectral index between 1.6 and 5 GHz as a function of orbital phase. The calculation was performed using ζ_rel = 0.15, ζ_B = 5 × 10^-5, p = 1.6, i = 60^°, and θ = 280^°. Observationally inferred spectral index values from van Loo et al. (2008) and Blomme et al. (2013) are shown for comparison − error bars have been computed by error propagation. The dashed horizontal line at spectral index of 0.6 corresponds to the canonical value for free-free emission from a single massive star wind.