A Two‐Zone Model for Type I X‐Ray Bursts on Accreting Neutron Stars

Abstract

We construct a two-zone model to describe hydrogen and helium burning on 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 stability analysis of Narayan & Heyl, including the delayed mixed burst regime. The results are also consistent with observations. At accretion rates M/M-Edd less than or similar to 0.1, helium burning via the well-known thin-shell thermal instability triggers bursts. As. M increases, however, the triggering mechanism evolves from the fast thermal instability to a slowly growing overstability involving both hydrogen and helium 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, drives oscillations with a period approximately equal to the hydrogen burning timescale. If these oscillations grow, the gradually rising temperature in the helium layer will eventually provoke a thin-shell thermal instability and hence a delayed mixed burst. For M/M-Edd less than or similar to 0.25, nuclear burning is stable and there are no bursts. Nearly all other theoretical models, including detailed time-dependent multizone calculations, predict that bursts should occur for all M/M-Edd less than or similar to 1, in conflict both with our results and with observations. We suggest that this discrepancy arises from the assumed strength of the hot CNO cycle breakout reaction O-15(alpha, gamma)Ne-19 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 O-15(alpha, gamma)Ne-19 rate is smaller than assumed in previous investigations.