POSTGLITCH RELAXATION OF THE CRAB PULSAR - EVIDENCE FOR CRUST CRACKING

The pattern of glitches and postglitch behavior observed for the Crab pulsar (Boynton et al. 1972; Demianski & Proszynski 1983; Lyne & Pritchard 1987; Lyne, Graham-Smith, & Pritchard 1992) is strikingly different from that observed for the Vela pulsar (Alpar et al. 1993). A key question is whether the differences can be understood on evolutionary grounds. An analysis of the Crab pulsar suggests that this is indeed the case. Thus, we propose that the comparatively modest (DELTAOMEGA/OMEGA approximately 10(-8)) and somewhat infrequent (approximately 6 yr inter-glitch intervals) Crab pulsar glitches are caused by starquakes induced by pulsar spin-down (Ruderman 1976; Baym & Pines 1971); we attribute its anomalous postglitch behavior (an occasional extended spin-up and a long-term response opposite in sign to that seen in the Vela pulsar (Lyne et al. 1992) to vortices transported inward during a quake, while the observed unexpected persistent change in angular acceleration, OMEGA(e) following a glitch represents the creation of a new vortex depletion region, as suggested by Alpar & Pines (1993).