23 YEARS OF CRAB PULSAR ROTATIONAL HISTORY

Between glitches, the rotational slowdown of the Crab pulsar is well d escribed by a power law with braking index 2.51+/-0.01. Six glitches o ccurred during the period from 1969 to 1993; the largest, in 1989, occ urred while observations were in progress and was observed in unpreced ented detail. The discontinuous changes in rotation at each glitch inc lude components whose effect decays over periods of between 1 and abou t 200 d; the main effect, however, is an increase of slowdown rate at the time of each glitch. This increase is persistent and cumulative, c ontributing 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 ve ry similar in form. In both there was a discontinuous increase in rota tion rate, followed by a decay with time constant of order 20 d. In th e 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 glitch es 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.