Apparent transverse velocity, β_s, of C7 as a function of the wave twist angle, φ, and transverse velocity, ${\beta }_{\mathrm{s}}^{\mathrm{g}}$ (in the frame of host galaxy), for a fixed viewing angle, θ = 6°, and longitudinal speeds of the fastest and slowest transverse waves, ${\beta }_{\mathrm{wave}}^{\mathrm{g}}=0.998$ and ${\beta }_{\mathrm{wave}}^{\mathrm{g}}=0.979$ (or the corresponding Lorentz factors, 15 and 3.5; full and dashed lines, respectively). The dotted horizontal line separates subluminal and superluminal apparent speeds.