Variation of ℒ with velocity distribution parameter σ for the model with a single isotropic Maxwellian (dotted lines), and for the mixed model in which most pulsars have an assumed semi-isotropic velocity distribution (solid lines). The colour coding indicates the pulsar sample: all 28 pulsars in our master list, the 27 pulsars remaining after removing PSR B1508+55, and the 19 youngest (τ < 10 Myr) pulsars.