2.6 Going Further2 The Pulsar Phenomenon2.4 Normal and Millisecond Pulsars

2.5 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 50 km s tex2html_wrap_inline1837 . The first direct evidence for this came from optical observations of the Crab pulsar 1968 [157], showing that the neutron star has a velocity in excess of 100 km s tex2html_wrap_inline1837 . Proper motions for about 100 pulsars have subsequently been measured largely by radio interferometric techniques [104, 24, 60, 70]. These data imply a broad velocity spectrum ranging from 0 to 1000 km s tex2html_wrap_inline1837 [105Jump To The Next Citation Point In The Article].

Such large velocities are perhaps not surprising, given the violent conditions which form neutron stars. Shklovskii [142] demonstrated that, if the explosion is only slightly asymmetric, an impulsive ``kick'' velocity of up to 1000 km s tex2html_wrap_inline1837 is imparted to the neutron star. As a result, a newly formed high-velocity pulsar quickly leaves its birth site close to the Galactic plane and on average it migrates to higher Galactic latitudes. This effect is seen most dramatically in Fig.  7, a dynamical simulation of the orbits of 100 neutron stars in a model of the Galactic gravitational potential.


Click on thumbnail to view image

Figure 7: A simulation following the motion of 100 pulsars in a model gravitational potential of our Galaxy for 200 Myr. The view is side-on i.e. the horizontal axis represents the galactic plane (30 kpc across) whilst the vertical axis represents tex2html_wrap_inline1819 kpc from the plane. This snapshot shows the initial configuration of young neutron stars. Click here to see the movie in action.

Using the proper motion data, recent studies have demonstrated that the mean birth velocity of normal pulsars is tex2html_wrap_inline1855 450 km s tex2html_wrap_inline1837 ([105, 96, 48, 64]; see, however, also [71, 69]). This is significantly larger than the millisecond pulsars -- recent studies suggest that their mean birth velocity is likely in the range tex2html_wrap_inline1929 km s tex2html_wrap_inline1837 [93, 47, 107Jump To The Next Citation Point In The Article]. The main reason for this difference surely lies in the fact that about 80% of the millisecond pulsars are members of binary systems (§ 2.4) which couldn't have survived if the neutron star had received a substantial kick velocities.

2.6 Going Further2 The Pulsar Phenomenon2.4 Normal and Millisecond Pulsars

image Binary and Millisecond Pulsars
D. R. Lorimer (dunc@mpifr-bonn.mpg.de)
© Max-Planck-Gesellschaft. ISSN 1433-8351
Problems/Comments to livrev@aei-potsdam.mpg.de