X-ray Superbursts : tackling an unsolved puzzle in the nucleosynthetic evolution of Neutron Stars

X-ray observations of compact objects by instruments such as RXTE, BeppoSax, Chandra have yielded a wealth of new information, yet models of processes deep in the Neutron Star (NS) crust remain surprisingly simple. At MSU we have developed the first detailed model of the NS crust with composition-dependent thermal transport and neutrino cooling. Further we couple to a full reaction network that begins with realistic rp-process ashes instead of a single nuclear species and incorporates Electron Captures (EC) through an analytic formulation allowing exact responses of EC thresholds to the electron chemical potential which rises as an accreted fluid element is pushed to greater depth. The heat release from EC on to excited states of daughter nuclei allows much more heat to be deposited in the crust than in earlier calculations, facilitating carbon ignition at depths that match the observed recurrence times and energetics of the mysterious X-ray superbursts-these are about a thousand times more energetic and long-lived than Type-IA X-ray Bursts on Neutron Stars.