Postglitch relaxation of the crab pulsar after its first four major glitches: The combined effects of crust cracking, formation of vortex depletion region and vortex creep

Following the application of vortex creep theory (Alpar et al. 1984; Alpar et al. 1993; Chau et al. 1993) to the postglitch behavior of the Vela pulsar, we extend the model to cover the postglitch behavior of the Crab pulsar (Alpar et al. 1994). We propose that the comparatively modest (Delta Omega/Omega similar to 10(-8)) and somewhat infrequent (similar to 6 yr interglitch intervals) Crab pulsar glitches are caused by crust cracking during starquakes induced by pulsar spin-down (Ruderman 1976; Baym & Pines 1971). We attribute the anomalous postglitch behavior (an occasional extended spin-up and a long-term response opposite in sign to that seen in the Vela pulsar [see Lyne, Graham-Smith, & Pritchard 1992]) to postglitch response to inward vortex bunching into newly formed vortex traps during a quake. The persistent shift in the angular acceleration Omega(c) following a glitch is attributed to the creation of a new vortex depletion region (Alpar & Pines 1993). The different postglitch behavior for the Crab and Vela pulsars can be understood on evolutionary grounds (Alpar et al. 1994).