Carbon Detonation and Shock‐Triggered Helium Burning in Neutron Star Superbursts

Abstract

The strong degeneracy of the ^{12}C ignition layer on anaccreting neutron star results in a hydrodynamic thermonuclear runaway,in which the nuclear heating time becomes shorter than the localdynamical time. We model the resulting combustion wave during thesesuperbursts as an upward-propagating detonation. We solve the reactivefluid flow and show that the detonation propagates through the deepestlayers of fuel and drives a shock wave that steepens as it travelsupward into lower density material. The shock is sufficiently strong onreaching the freshly accreted H/He layer that it triggers unstable^{4}He burning if the superburst occurs during the latter halfof the regular type I bursting cycle; this is likely the origin of thebright type I precursor bursts observed at the onset of superbursts. Thecooling of the outermost shock-heated layers produces a bright, ~0.1 s,flash that precedes the type I burst by a few seconds; this may be theorigin of the spike seen at the burst onset in 4U 1820-30 and 4U1636-54, the only two bursts observed with RXTE at high time resolution.The dominant products of the ^{12}C detonation are^{28}Si, ^{32}S, and ^{36}Ar. Gupta et al.showed that a crust composed of such intermediate-mass elements has alarger heat flux than one composed of iron-peak elements and helps bringthe superburst ignition depth into better agreement with values inferredfrom observations.