Proc. Neutrino Astrophysics - MPP Theory Group

54
Pulsar Velocities and Their Implications
A.G. Lyne
University of Manchester, Jodrell Bank, Macclesfield, Cheshire SK11 9DL, UK
In several respects, the Galactic distribution of pulsars mimics the distribution of population
I species, such as young stars, HII regions and supernova remnants. In particular, the
galactocentric radial distributions are very similar. However, the widths of the distributions
in distance from the Galactic plane, Z, differ by nearly a factor of 10. Gunn and Ostriker
(1970) [1] realised that this difference could result from high velocities which may be given
to pulsars at birth, possibly arising in the violence of their formation in supernova collapse.
It was several years later before the first measurement of the proper motion of a pulsar gave
direct evidence for this hypothesis [2]. Transverse velocities are now known for more than
100 pulsars and their high values are generally well established.
In this review, I describe briefly the techniques used in the measurement of pulsar velocities,
summarise the main features of the distribution of the velocities and discuss their
implications for the physics of the formation events and for their spatial distribution.
There are two main techniques which have been used for the determination of pulsar
velocities, in both cases providing the transverse components of the velocity only. These
techniques respectively involve the measurement of proper motion and of the velocity of the
interstellar scintillation pattern across the Earth. Proper motions are determined from highresolution
radio interferometry or from timing measurements. Like optical techniques, the
former involves measurement of the position of a pulsar relative to a nearby source over a
period of time. With modern instruments, position accuracy of 10 milliarcseconds (mas) is
readily achievable, so that only a few years are required to give proper motion errors of only a
few mas/yr [3, 4, 5, 6, 7]. Timing measurements can give similar precision in principle, but are
usually limited by irregularities in the rotation rate of the pulsar, known as “timing noise,”
which is predominant in young pulsars. However, timing is particularly useful for millisecond
pulsars which have very stable rotation and whose narrow pulses allow very high accuracy.
Estimates of pulsar transverse velocities Vt are then obtained from the proper motion µ
(mas/yr) and the distance D (kpc): Vt = 4.74µD km/s. The distance D is obtained from
the dispersion measure and an electron density model [8] which is based upon a number of
independent distance measurements. Nearly 100 such measurements of transverse velocity
are available.
The velocities of pulsars estimated from observations of the speed of the interstellar scintillation
patterns [9, 10] are considered rather less reliable than the more direct proper motion
measurements, mainly because it has recently been shown [11] that the values are systematically
low by an average factor of 2 because of a localisation of the scattering medium close to
the galactic plane. There are about 71 pulsars for which scintillation speeds are available [10],
although most of these pulsars have rather better proper motion measurements, leaving 27
which have only scintillation measurements. Combining both techniques gives a sample of
about 100 values of Vt, of which about 8 are upper limits.
From this sample, the mean transverse velocity is found to be 〈Vt〉 = 300 ± 30 km/s.
With a small number of exceptions, the velocity vectors shown in Fig. 1 demonstrate a clear
movement away from the galactic plane, consistent with pulsars being formed from massive,
young, population I stars close to the plane and receiving velocities of a few hundred km/s at