HIGH BIRTH VELOCITIES OF RADIO PULSARS

NEUTRON stars are usually born during the supernova explosion of a massive star. Any small asymmetry during the explosion can result in a substantial 'kick' velocity(1) to the neutron star. Pulsars (rapidly rotating, magnetized neutron stars) have long been known to have high space velocities(2,3), but new measurements of proper motion(4-6), adoption of a new distance scale for the pulsars(7) and the realization that some previous velocities were systematically low by a factor of 2 (ref. 8) have prompted us to reassess these velocities. Here, taking into account a strong selection effect that makes the observed velocities unrepresentative of those acquired at birth(9), we arrive at a mean pulsar birth velkocity of 450 +/- 90 km s(-1). This exceeds the escape velocity from binary systems, globular clusters and the Galaxy, and so will affect our understanding of the retention of neutron stars in these systems. Those neutron stars that are retained by the Milky Way will be distributed more isotropically than has been thought(10-12), which may result in a distribution like that of the gamma-ray burst sources.