Heating in the Accreted Neutron Star Ocean: Implications for Superburst Ignition

We perform a self-consistent calculation of the thermal structure in the crust of a superbursting neutron star. In particular, we follow the nucleosynthetic evolution of an accreted element from deposition into the atmosphere down to neutron drip density. We include temperature-dependent continuum electron capture rates and realistic sources of heat loss by thermal neutrino emission from the crust and core. We show that, in contrast to previous calculations, electron captures to excited states and subsequent gamma-emission significantly reduces the local heat loss due to weak-interaction neutrinos. Furthermore, temperature-sensitive (gamma, n) rates trigger further energy deposition as the distribution of nuclei evolves toward a lower neutron separation energy. Depending on the initial composition these reactions release up to a factor of ten times more heat at densities <10^11 g/cc than obtained previously. This heating reduces the ignition depth of superbursts. In particular, it reduces the discrepancy noted by Cumming et al. between the temperatures needed for unstable 12C ignition on timescales consistent with observations and the reduction in crust temperature from Cooper pair neutrino emission.