Knock-on processes in superfluid vortex avalanches and pulsar glitch statistics

A framework is presented for a statistical theory of neutron star glitches, motivated by the results emerging from recent Gross-Pitaevskii simulations of pinned, decelerating quantum condensates. It is shown that the observed glitch size distributions cannot be reproduced if superfluid vortices unpin independently via a Poisson process; the central limit theorem yields a narrow Gaussian for the size distribution, instead of the broad, power-law tail observed. This conclusion is not altered fundamentally when a range of pinning potentials is included, which leads to excavation of the potential distribution of occupied sites, vortex accumulation at strong pinning sites, and hence the occasional, abnormally large glitch. Knock-on processes are therefore needed to make the unpinning rate of a vortex conditional on the pinning state of its near and/or remote neighbours, so that the Gaussian size distributions resulting generically from the central limit theorem are avoided. At least two knock-on processes, nearest-neighbour proximity knock-on and remote acoustic knock-on, are clearly evident in the Gross-Pitaevskii simulation output. It is shown that scale-invariant (i.e. power-law) vortex avalanches occur when knock-on is included, provided that two specific relations hold between the temperature and spin-down torque. This fine-tuning is unlikely in an astronomical setting, leaving the overall problem partly unsolved. A state-dependent Poisson formalism is presented which will form the basis of future studies in this area.