Nuclear Data to Quantify Urca Cooling in Accreting Neutron Stars

Abstract

Neutron stars in Low Mass X-ray Binaries (LMXBs) can accrete matter onto their surface from the companion star. Transiently accreting neutron stars go through alternating phases of active accretion outbursts and quiescence. X-ray observations during the quiescence phase show a drop in X-ray luminosity with the time in quiescence. This is also inferred as the drop in surface temperature or the cooling of accreting neutron stars in quiescence. Analyzing these cooling curves reveals a great deal of information about the structure and composition of neutron stars. However, model-observation comparisons of such cooling curves are challenging - partly due to observational uncertainties, and partly due to incomplete knowledge of heating mechanisms during accretion outbursts. This situation is further exacerbated by the recent discovery of Urca cooling in the neutron star crust. These are cycles that alternate between electron-capture and beta-decay to produce a large flux of neutrinos and anti-neutrinos. These freely stream out of the star and carry energy with them, essentially cooling the neutron star crust without changing the composition. As a result, it is necessary to accurately quantify the strength of Urca cooling to constrain the heat sources in neutron star crusts and facilitate better model-observation comparisons of the cooling curves.