A Two-Zone Model for Type I X-ray Bursts on Accreting Neutron Stars

We construct a two-zone model to describe hydrogen and helium burning on the surface of an accreting neutron star and use it to study the triggering of type I X-ray bursts. Although highly simplified, the model reproduces all of the bursting regimes seen in the more complete global linear stability analysis of Narayan & Heyl (2003), including the regime of delayed mixed bursts. The results are also consistent with observations of type I X-ray bursts. At accretion rates ~ 25% of the Eddington limit, there is no instability or overstability, and there are no bursts