Long Type I X‐Ray Bursts and Neutron Star Interior Physics

Abstract

Two types of long-duration type I X-ray bursts have been discovered by long-term monitoringobservations of accreting neutron stars: superbursts and ``intermediate duration'' bursts. Weinvestigate the sensitivity of their ignition conditions to the interior thermal properties of theneutron star. First, we compare the observed superburst light curves to cooling models. Our fitsrequire ignition column depths in the range (0.5-3) × 10 12 g cm -2 and an energy release ≈2 × 10 17ergs g -1 . The implied carbon fraction is X C > 10%, constraining models of rp-process hydrogenburning. Neutrino emission and inwards conduction of heat lead to a characteristic surface fluenceof 10 42 ergs, in good agreement with observations. Next, we compare ignition models to observationsof superbursts. Consistent with our light-curve fits, carbon fractions X C ##IMG##[http://ej.iop.org/icons/Entities/gtrsim.gif] {gtrsim} 0.2 are needed to avoid stable burning at thelowest rates for which superbursts have been observed. Unstable carbon ignition at the observeddepths requires crust temperatures ≈6 × 10 8 K, which implies that neutrino emission from theinterior is inefficient, and the crust has a poor thermal conductivity. In particular, we cannotmatch observed superburst properties when Cooper pair neutrino emission from the crust is included.We conclude that an extra ingredient, for example additional heating of the accumulating fuel layer,is required to explain the observed properties of superbursts. If Cooper pair emission is lessefficient than currently thought, the observed ignition depths for superbursts imply that the crustis a poor conductor, and the core neutrino emission is not more efficient than modified Urca. Theobserved properties of helium bursts support these conclusions, requiring inefficient crustconductivity and core neutrino emission.