Accretion rate and the occurrence of multi-peaked X-ray bursts

Most Type I X-ray bursts from accreting neutron stars have a lightcurve with a single peak, but there is a rare population of faint bursts that are double or even triple peaked. Suggested mechanisms include polar ignition with equatorial stalling, or multi-step energy release; the latter being caused by hydrodynamic instabilities or waiting points in the nuclear reaction sequence. We present an analysis of the accretion rate dependence of the multi-peak bursts, and discuss the consequences for the various models. The observations pose particular challenges for the polar ignition mechanism given current models of ignition latitude dependence.