Successive X-ray bursts from accreting neutron stars

The evolution of a neutron star undergoing a series of thermonuclear flashes in its accreted hydrogen-rich layer has been numerically followed to determine the effects of the history of the neutron star's thermal and compositional structure on the properties of the emitted X-ray bursts. The burst characteristics have been studied for a range of mass accretion rates, CNO abundances in the accreted matter, and initial thermal states of the underlying neutron star core. It is found that the bursts exhibit erratic behavior, especially for low CNO metal abundances and cool neutron star cores, with the burst recurrence time scales varying by one to two orders of magnitude. The results of the calculations directly illustrate the importance of compositional and thermal inertia on burst behavior. A common characteristic of these models is the continued presence of a substantial amount of unburnt hydrogen in the accreted layer throughout the series of the X-ray burst events. Convective mixing during the quiescent phase leads to the inward transport of helium to high densities and eventually to the initiation of the next outburst. The resulting bursts can be weak and, in such cases, are characterized by short recurrence time scales (∼1-2 hr), low peak luminosities (∼0.1-0.2 times the Eddington value), and low α-values (∼20). The numerical results indicate that such mixing can lead to an irregular pattern of unstable nuclear burning, and it is suggested that this irregularity is manifested as the erratic bursting behavior seen in X-ray transients like 1608-522 and EXO 0748-67.