The 1969 Glitch in the Crab Pulsar Revisited

Glitches are important to understand the internal structure of neutron stars. They are studied using timing observations. The best studied neutron star in this respect is the Crab Pulsar. The first glitch recorded in this pulsar occurred in 1969 Sep, at an epoch when timing observations were still in their infancy, the regularity of the observations was relatively poor, and errors on the observations were relatively high. Lyne et al. (1993) analyzed most of the available data using modern techniques, and showed that this was a typical glitch of the Crab pulsar, with typical glitch parameters. This work analyses all available data, and shows that the 1969 event in the Crab pulsar is amenable to radically different interpretations. The Crab pulsar was timed by five different groups during this epoch, one at radio and the rest at optical frequencies. These data are available in the public domain, and have been analyzed using the TEMPO2 software. The 1969 event in the Crab pulsar can be better modeled as a typical glitch that was interrupted by a non-glitch speed-up event. This work also confirms the existence of a quasi-sinusoidal oscillation in the timing noise of the Crab pulsar, that was reported by Richards et al. (1970), but with a smaller period, and with its amplitude and period decreasing with time, like a chirp signal. Such a coherent oscillation has not been noticed so far in either the Crab or any other pulsar. This work provides an explanation for the post-glitch behavior of the Crab pulsar glitches of 1969 Sep and 2004 Nov, and similar glitches in other pulsars, in terms of the recently proposed non-glitch speed-up event. If true, then non-glitch speed-up events may not be as rare as believed earlier. This work argues that it is unlikely that the frequency and amplitude modulated sinusoidal variation in the timing noise is due to unmodeled planetary companions.