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

Abstract

We construct a two-zone model to describe H and He 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 Mdot < 0.1 Mdot_Edd, thermonuclear He burning via the well-known thin-shell thermal instability triggers bursts. As Mdot increases, however, the trigger mechanism evolves from the fast thermal instability to a slowly growing overstability involving both H and He burning. The competition between nuclear heating via the beta-limited CNO cycle and the triple-alpha process on the one hand, and radiative cooling via photon diffusion and emission on the other hand, drives oscillations with a period approximately equal to the H-burning timescale. If these oscillations grow, the gradually rising temperature at the base of the helium layer eventually provokes a thin-shell thermal instability and hence a delayed mixed burst. For Mdot > 0.25 Mdot_Edd, there is no instability or overstability, and there are no bursts. Nearly all other theoretical models predict that bursts should occur for all Mdot < Mdot_Edd, in conflict with both our results and observations. We suggest that this discrepancy arises from the assumed strength of the hot CNO cycle breakout reaction 15O(alpha,gamma)19Ne in these other models. That observations agree much better with the results of Narayan & Heyl and our two-zone model, both of which neglect breakout reactions, may imply that the true 15O(alpha,gamma)19Ne cross section is much smaller than assumed in previous investigations.

13 pages, 8 figures, accepted by ApJ

Cited by in corpus (12)

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• Turbulent Mixing in the Surface Layers of Accreting Neutron Stars
• Type I X-ray Bursts from the Neutron-star Transient XTE J1701-462
• The Latitude of Type I X-Ray Burst Ignition on Rapidly Rotating Neutron Stars
• Ignition latitude and the shape of Type I X-ray bursts
• Reaction Rate and Composition Dependence of the Stability of Thermonuclear Burning on Accreting Neutron Stars
• Hydrogen-Triggered Type I X-ray Bursts in a Two-Zone Model
• The Influence of Uncertainties in the 15O(alpha,gamma)19Ne Reaction Rate on Models of Type I X-Ray Bursts
• Generation of Type I X-ray Burst Oscillations by Unstable Surface Modes
• On the dependence of X-ray burst rate on accretion and spin rate
• Accretion rate and the occurrence of multi-peaked X-ray bursts
• Ignition column depths of helium-rich thermonuclear bursts from 4U 1728-34