Carbon detonation and shock-triggered helium burning in neutron star superbursts

The strong degeneracy of the C-12 ignition layer on an accreting neutron star results in a hydrodynamic thermonuclear runaway, in which the nuclear heating time becomes shorter than the local dynamical time. We model the resulting combustion wave during these superbursts as an upward-propagating detonation. We solve the reactive fluid flow and show that the detonation propagates through the deepest layers of fuel and drives a shock wave that steepens as it travels upward into lower density material. The shock is sufficiently strong on reaching the freshly accreted H/He layer that it triggers unstable He-4 burning if the superburst occurs during the latter half of the regular type I bursting cycle; this is likely the origin of the bright type I precursor bursts observed at the onset of superbursts. The cooling of the outermost shock-heated layers produces a bright, approximate to 0.1 s, flash that precedes the type I burst by a few seconds; this may be the origin of the spike seen at the burst onset in 4U 1820-30 and 4U 1636-54, the only two bursts observed with RXTE at high time resolution. The dominant products of the C-12 detonation are Si-28, S-32, and Ar-36. Gupta et al. showed that a crust composed of such intermediate-mass elements has a larger heat flux than one composed of iron-peak elements and helps bring the superburst ignition depth into better agreement with values inferred from observations.