Statistical studies of pulsar glitches

Shemar & Lyne have previously presented observations and an analysis of 32 glitches and their subsequent relaxations observed in a total of 15 pulsars . These data are brought together in this paper with those published by oth er authors. We show quantitatively how glitch activity decreases linearly w ith decreasing rate of slow-down. As indicated previously from studies of t he Vela pulsar, the analysis suggests that 1.7 per cent of the moment of in ertia of a typical neutron star is normally contained in pinned superfluid which releases its excess angular momentum at the time of a glitch. There i s a broad range of glitch amplitude and there is a strong indication that p ulsars with large magnetic fields suffer many small glitches while others s how a smaller number of large glitches. Transient effects following glitche s are very marked in young pulsars and decrease linearly with decreasing ra te of slow-down, suggesting that the amount of loosely pinned superfluid de creases with age. We suggest that the low braking index of the Vela and Cra b pulsars cannot be caused by a decreasing moment of inertia and should be attributed to step increases in the effective magnetic moment of the neutro n star at the glitches.