β-decay of ³³Mg to Quantify Urca Cooling in Accreting Neutron Stars

Quiescent X-ray observations of the cooling of transiently accreting neutron stars provide important clues about the structure and composition of the neutron star crusts. Once of the major sources of uncertainty in modeling this cooling behavior is the Urca cooling luminosity. It depends on the ground-state to ground-state β-decay transition strengths of exotic neutron rich nuclei that exist in the neutron star crust. ³³Mg is found to be a strong Urca cooling agent, partly due to its high mass fraction in the crust, and partly due to its strong β-decay ground-state branch with a log-ft value of 5.2(1). However, the recent assignment of a negative parity ground-state in ³³Mg make this a first-forbidden transition and such a low log-ft value seems anomalous. This was remeasured at the National Superconducting Cyclotron Laboratory (NSCL) using Total Absorption Spectroscopy with the SuN, NERO and BCS detector setup. The updated log-ft value of the ground-state branch for the β-decay of ³³Mg is 7.0 with a 1-σ lower limit of 6.3, consistent with the first-forbidden nature of this transition. This further corresponds to a two orders of magnitude reduction in the Urca cooling luminosity for a neutron star crust composed of Mass A = 33 ashes. This update facilitates more accurate model-observation comparisons of accreting neutron star crusts.