Hydrodynamic simulations of thermonuclear bursts on accreting neutron stars: a showcase of burning regimes

Abstract Type I X-ray bursts are one of the most frequent and energetic phenomena observed in the Galaxy. Numerical studies of accreting neutron stars undergoing type I X-ray bursts provide a tool to understand steady burning on the surface of neutron stars and calculate the properties of regular bursts, as well as rare and energetic bursts at low accretion rates. We model type I X-ray bursts for different physical model parameters, using the MESA (Modules of Experiments in Stellar Astrophysics) code. We determine how mass, metallicity, base luminosity and mass-accretion rate affect the observed recurrence time of bursts. We also look at the accretion rates associated with the transition between stable H and He burning, He ignition with mixed H/He burning, He ignition in a H-depleted layer, shallow H ignition with He accumulation and deep H ignition followed by He ignition. Depending on the accretion rate, the observed recurrence time shows a different correlation with increasing metallicity. For the first time, we identify the igniting fuel numerically, by monitoring the CNO and triple-alpha reaction fluxes during the burst, instead of the conventional timescale of H depletion through CNO burning. From the CNO and triple-alpha reaction fluxes we find a unique pattern for each case, which we use to distinguish between each bursting regime.