Only a few years ago scientists thought that neutron stars had
velocities of 100 - 200 km s
(see, e.g., Lyne, Anderson, and
Salter 1982). But recent studies show (Lyne & Lorimer 1994; Frail,
Goss, & Whiteoak 1994) that as much as 50% of neutron stars have
velocities 
km s. These velocities are so high that
these neutron stars escape from the Galaxy and produce a distant,
previously unknown Galactic "corona."
The evidence that many neutron stars have high velocities comes from
two independent directions. In the first case, long-wavelength radio
observations have discovered that many young radio pulsars are
associated with young (
yrs) supernova remnants
(Frail, Kassim, and Weiler 1994; Frail, Goss, and Whiteoak 1994).
Sometimes the young pulsar lies within the shell-like supernova
remnant; sometimes it is passing through the shell, as the spectacular
radio image of the "duck" supernova remnant and pulsar PSR1757-24
reveals (see Figure 4); and sometimes the young pulsar is associated
only with a comet-like "plerion," or filled remnant. In every case the
pulsar lies far from the center of the remnant. These offsets imply
median transverse velocities 
km s, with 
1/3 of
the neutron stars having transverse velocities 
km s
(see Figure 5) (Frail, Goss, and Whiteoak 1994). Optical observations
of bow shocks have also shown that some older pulsars have transverse
velocities 
km s (see, e.g., Cordes, Romani, and Lundgren
1993).
In the second case, a new model for the electron density in the Milky
Way and a greater understanding of an important observational bias that
affects the determination of pulsar velocities has dramatically
increased the velocities inferred for older pulsars. The new electron
density model shows that the distance to, and therefore the transverse
velocity of, nearby pulsars was underestimated by about a factor of two
in previous models. The observational bias that affects the
determination of pulsar velocities arises because young radio pulsars
are born close to the Galactic plane, and move rapidly away from it if
their velocity is high. After some time, the pulsars that remain
within detectable range are mostly those with small velocities. The
strength of the bias is illustrated by the fact that the mean of the
distribution of transverse velocities is 
km s for
pulsars with spindown ages 
3 Myr, whereas it is 
km s for pulsars with 
70 Myr.
Recent studies that incorporate these discoveries yield median neutron
star total velocities 
km
s, with as many as half of all neutron stars having velocities
km s (Lyne and Lorimer 1994; Chernoff 1995).
These results have revolutionized our understanding of the spatial
distribution of neutron stars. Since the escape velocity from the
Galaxy is 
km s in the Galactic bulge, where most
neutron stars are born, and 
km s in the solar
neighborhood, all of these high velocity neutron stars will escape from
the Milky Way. They form a distant, previously unknown "corona" around
the Milky Way (see Figure 6). This distant corona contains an ample
population of sources which appear isotropic when viewed from the
Earth.