Multi-Zone Models of Superbursts from Accreting Neutron Stars

Abstract

Superbursts are rare and energetic thermonuclear carbon flashes observed to occur on accreting neutron stars. We create the first multi-zone models of series of superbursts using a stellar evolution code. We self-consistently build up the fuel layer at different rates, spanning the entire range of observed mass accretion rates for superbursters. For all models light curves are presented. They generally exhibit a shock breakout, a precursor burst due to shock heating, and a two-component power-law decay. Shock heating alone is sufficient for a bright precursor, that follows the shock breakout on a short dynamical time scale due to the fall-back of expanded layers. Models at the highest accretion rates, however, lack a shock breakout, precursor, and the first power law decay component. The ashes of the superburst that form the outer crust are predominantly composed of iron, but a superburst leaves a silicon-rich layer behind in which the next one ignites. Comparing the model light curves to an observed superburst from 4U 1636-53, we find for our accretion composition the best agreement with a model at three times the observed accretion rate. We study the dependence on crustal heating of observables such as the recurrence time and the decay time scale. It remains difficult, however, to constrain crustal heating, if there is no good match with the observed accretion rate, as we see for 4U 1636-53.