Posterior distributions of v_kick versus ẽ for the first six BNSs (rows), following either the GC-isotropic (two left columns) or LSR-isotropic (two right columns) estimations of ẽ, as shown in Fig. 9. For each isotropy assumption, we computed the posterior using the results from the exponential disc (left column) or Gaussian annulus (right column) prior, as shown in Fig. 10. The posteriors are shown in 2D histograms with ẽ bins of 0.05 and v_kick bins of 10 km/s, and are normalised so that the (linear) colour scale shows the probability density (ρ) in units of 10⁻² s/km.