Fast neutrino cooling in the accreting neutron star MXB 1659-29

TL;DR

This paper investigates the rapid neutrino cooling in the neutron star MXB 1659-29, exploring how core reactions, superfluid gaps, and the nuclear symmetry energy influence the star's cooling behavior and inferred properties.

Contribution

It introduces detailed neutron star models with variable equation of state parameters and superfluid gaps to explain observed neutrino luminosity and cooling mechanisms in MXB 1659-29.

Findings

Neutrino luminosity consistent with direct Urca reactions in ~1% of the core.

Larger symmetry energy slope L requires superfluid suppression of Urca in low mass stars.

Models with reduced Urca normalization can explain observations with larger emitting volumes.

Abstract

Modelling of crust heating and cooling across multiple accretion outbursts of the low mass X-ray binary MXB 1659-29 indicates that the neutrino luminosity of the neutron star core is consistent with direct Urca reactions occurring in $\sim 1\%$ of the core volume. We investigate this scenario with neutron star models that include a detailed equation of state parametrized by the slope of the nuclear symmetry energy $L$, and a range of neutron and proton superfluid gaps. We find that the predicted neutron star mass depends sensitively on $L$ and the assumed gaps. We discuss which combinations of superfluid gaps reproduce the inferred neutrino luminosity. Larger values of $L\gtrsim 80\ {\rm MeV}$ require superfluidity to suppress dUrca reactions in low mass neutron stars, i.e. that the proton or neutron gap is sufficiently strong and extends to high enough density. However, the largest gaps give masses near the maximum mass, making it difficult to accommodate colder neutron stars. We consider models with reduced dUrca normalization as an approximation of alternative, less efficient, fast cooling processes in exotic cores. We find solutions with a larger emitting volume, providing a more natural explanation for the observed neutrino luminosity, provided the fast cooling process is within a factor of $\sim 1000$ of dUrca. The heat capacities of our models span the range from fully-paired to fully-unpaired nucleons meaning that long term observations of core cooling could distinguish between models. We discuss the impact of future constraints on neutron star mass, radius and the density dependence of the symmetry energy.