Linear polarisation degree P′_L induced by Compton scattering, for photons with an initial polarisation degree P_L of 0, 0.2, 0.5 and 0.9. P′_L varies strongly with the scattering direction (with φ − γ being the azimuthal scattering angle relative to the initial polarisation direction). For P_L = 0 (grey), P′_L is independent of the azimuthal direction by definition. The energy dependence in the 2–8 keV range is negligible.