Anti-glitches in accreting pulsars from superfluid vortex avalanches

Three sudden spin-down events, termed `anti-glitches', were recently discovered in the accreting pulsar NGC 300 ULX-1 by the \textit{Neutron Star Interior Composition Explorer} (NICER) mission. Unlike previous anti-glitches detected in decelerating magnetars, these are the first anti-glitches recorded in an accelerating pulsar. One standard theory is that pulsar spin-up glitches are caused by avalanches of collectively unpinning vortices that transfer angular momentum from the superfluid interior to the crust of a neutron star. Here we test whether vortex avalanches are also consistent with the anti-glitches in NGC 300 ULX-1, with the angular momentum transfer reversed. We perform $N$-body simulations of up to $5 \times 10^{3}$ pinned vortices in two dimensions in secularly accelerating and decelerating containers. Vortex avalanches routinely occur in both scenarios, propagating inwards and outwards respectively. The implications for observables, such as size and waiting time statistics, are considered briefly.