Pulsar Spin--Velocity Alignment: Further Results and Discussion
  The reported alignment between the projected spin-axes and proper motion
directions of pulsars is revisited in the light of new data from Jodrell Bank
and Effelsberg. The present investigation uses 54 pulsars, the largest to date
sample of pulsars with proper-motion and absolute polarisation, to study this
effect. Our study has found strong evidence for pulsar spin-velocity alignment,
excluding that those two vectors are completely uncorrelated, with >99%
confidence. Although we cannot exclude the possibility of orthogonal
spin-velocity configurations, comparison of the data with simulations shows
that the scenario of aligned vectors is more likely than that of the orthogonal
case. Moreover, we have determined the spread of velocities that a spin-aligned
and spin-orthogonal distribution of kicks must have to produce the observed
distribution of spin-velocity angle offsets. If the observed distribution of
spin-velocity offset angles is the result of spin-aligned kicks, then we find
that the distribution of kick-velocity directions must be broad with
{\sigma}_v~30\degree if the orthogonal-kick scenario is assumed, then the
velocity distribution is much narrower with {\sigma}_v<10\degree. Finally, in
contrast to previous studies, we have performed robustness tests on our data,
in order to determine whether our conclusions are the result of a statistical
and/or systematic bias. The conclusion of a correlation between the spin and
velocity vectors is independent of a bias introduced by subsets in the total
sample. Moreover, we estimate that the observed alignment is robust to within
10% systematic uncertainties on the determination of the spin-axis direction
from polarisation data.
