Long Type I X-ray Bursts and Neutron Star Interior Physics
نویسنده
چکیده
Superbursts are very energetic Type I X-ray bursts discovered in recent years by long term monitoring of Xray bursters, believed to be due to unstable ignition of carbon in the deep ocean of the neutron star. A number of intermediate duration bursts have also been observed, probably associated with ignition of a thick helium layer. We investigate the sensitivity of these long X-ray bursts to the thermal profile of the neutron star crust and core. We first compare cooling models of superburst lightcurves with observations, and derive constraints on the ignition mass and energy release. Despite the large uncertainties associated with the distance to each source, these parameters are quite well constrained in our fits. For the observed superbursts, we find ignition column depths in the range 0.5–3× 1012 g cm−2, and energy release ≈ 2× 1017 erg g−1. This energy release implies carbon fractions of XC > 10%, constraining models of rp-process hydrogen burning. We then calculate ignition models for superbursts and pure helium bursts, and compare to observations. We show that achieving unstable ignition of carbon at accretion rates less than 0.3 of the Eddington rate requires XC & 0.2, consistent with our lightcurve fits. Most importantly, we find that when Cooper pairing neutrino emission in the crust is included, the crust temperature is too low to support unstable carbon ignition at column depths of ∼ 1012 g cm−2. Some additional heating mechanism is required in the accumulating fuel layer to explain the observed properties of superbursts. If Cooper pair emission is less efficient than currently thought, the observed ignition depths for superbursts imply that the crust is a poor conductor, and the core neutrino emission is not more efficient than modified URCA. The observed properties of helium bursts support these conclusions, requiring inefficient crust conductivity and core neutrino emission. Subject headings: accretion, accretion disks-X-rays:bursts-stars:neutron
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