7 Timing irregularities

Summary

On a time scale of some days, all pulsars show a remarkable uniformity of rotation rate. This is not surprising, since uniform rotation is exactly what is expected of a spinning body with a large stable moment of inertia and which is isolated in space. The angular momentum of the star can only change through the slowdown torque of the magnetic dipole radiation, or an associated material outflow, or, for the accretion-powered X-ray pulsars, the accelerating torque of in-falling material carrying angular momentum of a binary system. The effects on the radio pulsar are usually smooth and predictable: however, some very interesting irregularities in pulsar rotation have been observed, which are related to changes both within the interior of the neutron star and outside it.

These internal changes in pulsars appear to be spasmodic adjustments towards a slowly changing equilibrium state. For example, the rapidly rotating star will be appreciably oblate, and the equilibrium ellipticity will decrease during the slowdown; the crust is however extremely rigid and can only adjust to the changing ellipticity in a series of steps. The corresponding changes in moment of inertia might be large enough to be observable, since conservation of angular momentum will result in their being seen as steps in rotation rate. This effect is not in fact large enough to account for most of the observed repeated steps in rotation rate, and these are instead attributed to an interchange of angular momentum between the crust and the fluid interior.