23 years of Crab pulsar rotational history

Between glitches, the rotational slowdown of the Crab pulsar is well described by a power law with braking index 2.51 ± 0.01. Six glitches occurred during the period from 1969 to 1993; the largest, in 1989, occurred while observations were in progress and was observed in unprecedented detail. The discontinuous changes in rotation at each glitch include components whose effect decays over periods of between 1 and about 200 d; the main effect, however, is an increase of slowdown rate at the time of each glitch. This increase is persistent and cumulative, contributing an overall increase of 0.07 per cent in the slowdown rate over the 23 years. The two largest glitches, in 1975 and 1989, were very similar in form. In both there was a discontinuous increase in rotation rate, followed by a decay with time constant of order 20 d. In the 1989 glitch, part of the step increase was resolved as an asymptotic exponential rise with a time constant of 0.8 d. In four of the glitches there was also a delayed increase in rotation rate, in the form of another asymptotic exponential rise with a time constant of order 200 d.