Proc. Neutrino Astrophysics - MPP Theory Group

Figure 1: The motions of pulsars relative to the galactic plane. Pulsars are represented as
filled circles in galactic longitude and Z-distance, the tail representing its approximate motion
in the past 1 Myr.
the same time. How large are the typical space velocities? Unfortunately, there is a selection
effect visible in the data in which high velocity pulsars quickly move away from the plane of the
Galaxy and become undetectable, leaving an excess of low-velocity pulsars. This effect can be
minimised by considering the velocity distribution of only the youngest pulsars, which cannot
have moved far since birth. The 29 pulsars with characteristic age of less than 3 Myr give
〈Vt〉 = 350 ± 70 km/s. Monte Carlo simulations show that the mean space velocity required
to give such an observed transverse velocity distribution is 〈Vs〉 = 450 ± 100 km/s [12].
There are two other pieces of evidence which support such high velocities: the increase
of the mean distance of pulsars from the plane as a function of characteristic age and the
movement of a small number of pulsars from the centres of the putative supernova remnants
associated with their birth [12].
Similar analyses for the 13 millisecond pulsars with proper motion measurements give
〈Vt〉 = 88±19 km/s and 〈Vs〉 = 130±30 km/s, showing them also to be a rather high-velocity
population. Not surprisingly, no migration from the galactic plane is seen, because of their
large ages. These velocities probably represent the low-velocity end of the “kick” spectrum—
higher velocity kicks are likely to disrupt any binary system, preventing any millisecond pulsar
formation by accretion spin-up in a binary system.
Clearly, the high velocities explain why only a few percent of pulsars are in binary systems,
compared with the high proportion of the likely progenitor stars in binaries. However, the
high velocities pose something of a problem for the retention of pulsars in globular clusters:
the velocities mostly exceed the cluster escape velocities and yet there is a substantial pulsar
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