The origin of pulsar velocities

Abstract

This thesis describes the planning, observations and analysis of a pulsar proper motion survey carried out with the Parkes-Tidbinbilla Interferometer from September 1986 until November 1988. It describes the interpretation of the results, and a self-consistent model of pulsar and massive binary system evolution. We have determined the proper motions of 6 radio pulsars. The accuracy of our measurements is high, and the derived proper motion between two reference sources of 0.5 ± 1 mas yr⁻¹ indicates that systematic errors affecting our derived proper motions are small. Our results confirm that: • Pulsars are high velocity objects. • There is a correlation between the velocity and magnetic field strength of radio pulsars. • Most pulsar proper motion vectors are directed away from the galactic plane. The proper motion of the Vela pulsar is discussed. We conclude that the region of enhanced emission, Vela-X, is not pulsar-driven, and that the Vela SNR has expanded asymmetrically since its birth. The trigonometric parallax of PSR 1451-68 has been determined by our observations. The parallax of 2.2 ± 0.3 mas yields a value of 0.19 ± 0.03 cm⁻³ for the column density of free electrons along the line of sight to this pulsar. A model for the origin of pulsar velocities and the evolution of massive binary systems is presented. In such systems, both stars end their life as a supernova and leave a neutron star remnant which may be observable as a pulsar. In the model, radio pulsars receive impulsive kick velocities at birth of order 200 km s⁻¹ . Contrary to conventional assuptions, pulsar magnetic fields are assumed not to decay, but are only reduced by the accretion of matter. We suggest that massive binary systems with orbital periods of a few months prior to the common envelope phase do not undergo spiral-in, but remain wide until the second supernova explosion. If this is so then it follows that: • Very few binary pulsars are formed. • Two populations of pulsars arise, one of which has low magnetic field strengths (1010 -1011 G) and low velocities ( rv 30 km s⁻¹ ). These pulsars have had their fields reduced by accretion in a massive binary system before being released with velocities similar to their orbital speeds. The other population is born with high fields (1012-1013 G) and high velocities ( rv 200 km s⁻¹ ) similar to the kick they received at birth. These pulsars come from either single stars or a supernova which disrupts a binary system. • The orbital eccentricities of the Be X-ray binaries are high. • Low-mass X-ray binaries and low-mass binary pulsars can be formed via direct supernova-collapse of massive stars. We review this model in chapter 6 with reference to the results of our observations and the newly-discovered binary pulsar PSR 1820-11. We find: • Pulsars with small characteristic ages are poor tools for determining the pulsar magnetic field decay timescale. • The only two pulsars with large characteristic ages and small errors in their z-velocities both have initial z-heights far more compatible with an infinite decay timescale than with one of several million years as is often postulated. • The velocity magnetic-moment correlation is almost certainly real. • Pulsars with high magnetic field strengths have initial velocities of order 200 km s⁻¹ . The percentage of pulsars born with low velocities is determined by the field decay timescale. For small field decay timescales, about 50 % of pulsars are born with velocities < 50 km s-1 . However, for infinite decay timescales, this fraction is <15 % and most pulsars have velocities of order 200 km s⁻¹ . • The binary pulsar PSR 1820-11 cannot be explained by conventional evolutionary scenarios, but the existence of a small number of such objects is consistent with our model. • If explosions are symmetric, we expect a much greater number of single pulsars with similar spin and magnetic field characteristics to PSR 1913+16 than are observed. Show more