Abstract
Two types of non-secular changes in pulsar frequency f have been reported. One is a sudden small frequency jump Δf in both NP 0532 (the Crab pulsar)1,2 and PSR 0833 (the Vela pulsar)3,4. The other is a slow, small amplitude “wobble” of frequency v in the Crab pulsar frequency5. A number of explanations for the jumps have been put forward, based on the model of a pulsar as a rotating neutron star: (1) the near passage of a planet in an orbit of high eccentricity (unpublished work of Michel); (2) a fluid instability caused by the crust of the neutron star rotating more slowly than its core6; (3) a sudden relaxation of strains which have built up in the stellar crust (starquake)7,8. Suggested mechanisms for the wobble include: (a) induced motion by yet another planet5; (b) free precession of a non-spherical star (unpublished work of Chiuderi and Occhionero); (c) normal mode vibrations of quantized vortex lines within the rotating superfluid core9.
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