Thermal Evolution of Accreting Neutron Stars. II. Long X-Ray Bursts as a Probe into the Interior

The possibility of distinguishing various internal thermal states of accreting neutron stars on the basis of the observed properties of X-ray bursts is demonstrated. For neutron stars with low core temperatures the burst properties are found to separate clearly at an accretion rate of roughly 0.01 of the critical rate related to the Eddington luminosity. For faster accretion, shell flashes produce normal type I X-ray bursts lasting up to a few tens of seconds, whereas for slower accretion long-duration bursts are expected with peak luminosities at two different levels. For a hot-core neutron star, such long bursts occur only within a narrow range around an accretion rate of about 0.005 of the critical rate; only the higher level of peak luminosity occurs. The relevance of these results to existing observations is discussed.