Context : Glitches are rare spin-up events that punctuate the smooth slow-down of the rotation of pulsars .
For the Vela pulsar and PSR J0537 - 6910 , the glitch sizes and the times between consecutive events have clear preferred scales ( Gaussian distributions ) , contrary to the handful of other pulsars with enough glitches for such a study .
Moreover , PSR J0537 - 6910 is the only pulsar showing a strong positive correlation between the size of each glitch and the waiting time until the following one .

Aims : We attempt to understand this behaviour through a detailed study of the distributions and correlations of glitch properties for the eight pulsars with at least ten detected glitches .

Methods : We model the distributions of glitch sizes and times between consecutive glitches for this sample .
Monte Carlo simulations are used to explore two hypotheses that could explain why the correlation is so much weaker in other pulsars than in PSR J0537 - 6910 .

Results : We confirm the above results for the Vela pulsar and PSR J0537 - 6910 , and verify that the latter is the only pulsar with a strong correlation between glitch size and waiting time to the following glitch .
For the remaining six pulsars , the waiting time distributions are best fitted by exponentials , and the size distributions either by power laws , exponentials , or log-normal functions .
Some pulsars in the sample yield significant Pearson and Spearman coefficients ( r _ { p } and r _ { s } ) for the aforementioned correlation .
Moreover , for all except the Crab , both coefficients are positive .
For each coefficient taken separately , the probability of this happening by chance is 1 / 16 .
Our simulations show that the weaker correlations in pulsars other than PSR J0537 - 6910 can not be due to missing glitches too small to be detected .
We also tested the hypothesis that each pulsar may have two kinds of glitches , namely large , correlated ones and small , uncorrelated ones .
The best results are obtained for the Vela pulsar , which exhibits a correlation with r _ { p } = 0.68 ( p -value = 0.003 ) if its 2 smallest glitches are removed .
The other pulsars are harder to accommodate under this hypothesis , but their glitches are not consistent with a pure uncorrelated population either .
We also found that all pulsars in our sample , except the Crab , are consistent with the previously found constant ratio between glitch activity and spin-down rate , \dot { \nu } _ { g } / | \dot { \nu } | = 0.010 \pm 0.001 , even though some of them have not shown any large glitches .

Conclusions :

To explain these results , we speculate that , except in the case of the Crab pulsar , all glitches draw their angular momentum from a common reservoir ( presumably a neutron superfluid component containing \approx 1 \% of the star ’ s moment of inertia ) , but two different trigger mechanisms could be active , a more deterministic one for larger glitches and a more random one for smaller ones .