Why pulsars rotate and move : kicks at birth

RADIO pulsars are thought to born with spin periods of 0.02−0.5 s 1,2,3 and space velocities of 100-1000 km s −14,5 , and they are inferred to have initial dipole magnetic fields of 10 11 − 10 13 G 2. The average space velocity of a normal star in the Milky Way is only 30 km s −1 , which means that pulsars must receive a substantial 'kick' at birth. Here we propose that the birth characteristics of pulsars have a simple physical connection with each other. Magnetic fields maintained by differential rotation between the core and envelope of the progenitor would keep the whole star in a state of approximately uniform rotation until 10 years before the explosion. Such a slowly rotating core has 1000 times less angular momentum than required to explain the rotation of pulsars. Although the specific physical process that 'kicks' the neutron star at birth has not been identified, unless its force is exerted exactly head-on 6 , it will also cause the neutron star to rotate. We identify this process as the origin of the spin of pulsars. Such kicks will cause a correlation between the velocity and spin vectors of pulsars. We predict that many neutron stars are born with periods longer than 2 s, and never become radio pulsars. A common interpretation of the magnetic fields and spins of pulsars appeals to the magnetic fields and rotation of the main sequence stars from which neutron stars descend 7. Compressing the Sun to the size of a neutron star, conserving (surface dipole) magnetic flux and angular momentum, would produce a neutron star of surface dipole field B s ∼ 10 11 G and spin period P ∼ 0.03 s, reminiscent of observed values. For this explanation to work, the small core of the supernova progenitor must be able to spin freely in its large slowly rotating envelope. The magnetic fields inside stars are unobservable, but rotation is, in the case of the Sun. The rotation below the convective envelope is essentially uniform 8,9 , with measured degrees of differential rotation well below the 30% level seen at the solar surface. The spin-down torque due to the solar wind has apparently been transmitted very effectively to the core. Such a low degree of differential rotation is incompatible 10 with the hydrodynamic angular momentum transport mechanisms commonly used in stellar evolution calculations 11. Simple estimates …