SPIN-UP AND RECOVERY IN THE 1989 GLITCH OF THE CRAB PULSAR

THE rotation of the Crab pulsar, as measured by pulsed radio and optical emission, has been monitored almost continuously since its discovery in 1968. A steady decrease in the rotation rate has been interrupted at intervals of about five years by glitches: discontinuous increases in rotation speed followed by partial recovery in the form of an exponential return to a new slowdown rate on a timescale of about 20 days. The recovery is usually interpreted as the re-establishment of stable differential rotation between the neutron star crust and part of its superfluid interior. Here we describe the largest glitch so far recorded in the Crab pulsar1. It occurred in 1989 while observations were being made, and the recovery was followed in unprecedented detail. The spin-up itself was also partly resolved in time, and the whole event, glitch plus recovery, is now seen to include three distinct exponential components, with timescales of 1, 20 and 300 days. The physical interpretation of these distinct components is unclear. The 1989 glitch, like the 1975 one2, caused a permanent increase in the slowdown rate, of a magnitude that is difficult to reconcile with current neutron star models.