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The de-excited energy of electron capture in accreting neutron star crusts

When a daughter nucleus produced by electron capture takes part in a level transition from an excited state to its ground state in accreting neutron star crusts, thermal energy will be released and heat the crust, increasing crust temperature and changing subsequent carbon ignition conditions. Previous studies show that the theoretical carbon ignition depth is deeper than the value inferred from observations because the thermal energy is not sufficient. In this paper, we present the de-excited energy from electron capture of rp-process ash before carbon ignition, especially for the initial evolution stage of rp-process ash, by using a level-to-level transition method. We find the theoretical column density of carbon ignition in the resulting superbursts and compare it with observations. The calculation of the electron capture process is based on a more reliable level-to-level transition, adopting new data from experiments or theoretical models (e.g., large-scale shell model and proton-neutron quasi-particle random phase approximation). The new carbon ignition depth is estimated by fitting from previous results of a nuclear reaction network. Our results show the average de-excited energy from electron capture before carbon ignition is ∼0.026 MeV/u, which is significantly larger than the previous results. This energy is beneficial for enhancing the crust's temperature and decreasing the carbon ignition depth of superbursts.