Photodisintegration-triggered Nuclear Energy Release in Superbursts

Superbursts from accreting neutron stars present a unique opportunity to probe nuclear processes at densities (rho) ~~ 10^9 g cm^-3 and temperatures T > 10^9 K. Cumming & Bildsten showed that these 10^42 erg bursts are most likely triggered by unstable ignition of a small amount of carbon in a sea of heavy nuclei made during prior rp-process burning of hydrogen and helium. We show here that the high temperatures reached during superbursts lead to photodisintegration reactions, which trigger conversion of these heavy nuclei into iron group elements. The neutron richness of the heavy nuclei allow ((gamma), n) reactions to trigger the photodisintegration runaway. Because the heavy elements are initially far from the minimum in binding energy, the nuclear energy release in this process can easily dominate that from the fusion of carbon. To our knowledge, this is the only cosmic phenomenon where the destruction of heavy nuclei into lighter nuclei represents the main source of energy.