Sketch of Vela X−1 from Grinberg et al. (2017) showing the accretion and photoionisation wakes. The blue circle represents the donor star HD 77851, while the pulsar is hidden in the accretion wake. The grey arrow indicates the verse of the rotation of the binary system. At the orbital phase φ_orb ≈ 0.75, the observer is looking at the system from the right, so that the line of sight (dashed line) is crossing the photoionisation wake.