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 15 O(α, γ) 19 Ne on the occurrence of type I X-ray bursts on accreting neutron stars. For f rp ≲ 0.1, where f rp is a dimensionless factor by which we multiply the 15 O(α, γ) 19 Ne reaction rate of Caughlan & Fowler, our model predicts that bursts should occur only for accretion rates below a critical value ≈ 0.3 Edd . This agrees with observations. For larger values of f rp , including the standard choice f rp = 1, the model switches to a new regime in which bursts occur all the way up to ≈ Edd . Since the latter regime disagrees with observations, we suggest that the true 15 O(α, γ) 19 Ne reaction rate is lower than usually assumed.