On the Physics of Type I X-ray Bursts on Accreting Neutron Stars at High Accretion Rates

We investigate the effect of the hot CNO cycle breakout reaction 15O(α,γ)19Ne on the occurrence of type I X-ray bursts on accreting neutron stars. For frp . 0.1, where frp is a dimensionless factor by which we multiply the 15O(α,γ)19Ne reaction rate of Caughlan & Fowler (1988), our model predicts that bursts should occur only for accretion rates Ṁ below a critical value ≈ 0.3ṀEdd. This agrees with observations. For larger values of frp, including the standard choice frp = 1, the model switches to a new regime in which bursts occur all the way up to Ṁ ≈ ṀEdd. Since the latter regime disagrees with observations, we suggest that the true 15O(α,γ)19Ne reaction rate is lower than usually assumed. Subject headings: dense matter — nuclear reactions, nucleosynthesis, abundances — stars: neutron — X-rays: binaries — X-rays: bursts