2.8 Pulsar velocities
Pulsars have long been known to have space velocities at least an order of magnitude larger than those
of their main sequence progenitors, which have typical values between 10 and
. The first direct
evidence for large velocities came from optical observations of the Crab pulsar [321], showing that the
neutron star has a velocity in excess of
. Proper motions for 233 pulsars have subsequently
been measured largely by radio timing and interferometric techniques [194, 24, 96, 115, 122
]. These data
imply a broad velocity spectrum ranging from 0 to over
[201
].
Such large velocities are perhaps not surprising, given the violent conditions under which neutron stars
are formed. Shklovskii [282] demonstrated that, if the explosion is only slightly asymmetric, an impulsive
“kick” velocity of up to
can be imparted to the neutron star. In addition, if the neutron star
progenitor was a member of a binary system prior to the explosion, the pre-supernova orbital velocity will
also contribute to the resulting speed of the newly-formed pulsar. As Figure 10
illustrates, high-velocity
pulsars born close to the Galactic plane quickly migrate to higher Galactic latitudes. Given such a broad
velocity spectrum, as many as half of all pulsars will eventually escape the Galactic gravitational
potential [201, 71].
The distribution of pulsar velocities remains an issue of contention. For example, Monte Carlo
simulations by Arzoumanian et al. [8
] strongly favour a bimodal distribution with low and high velocity
peaks. On the other hand, a recent study [122] found the mean birth velocity of normal pulsars to be
consistent with a Maxwellian distribution with a mean of
. Regardless of the form of the
distribution, however, we can say that the mean velocity of young pulsars is significantly larger than the
range for millisecond and binary pulsars [178, 70, 205
]. The main reasons for
the lower velocities are the fact that the kick must have been small to avoid disruption, and
the surviving neutron star has to pull the companion along with it, thus slowing the system
down.