Probing dense matter physics with transiently-accreting neutron stars: the case of source MXB 1659-29

TL;DR

This paper models the thermal evolution of the neutron star MXB 1659-29, constraining its core superfluidity and the nuclear symmetry energy by comparing detailed equations of state with observational data.

Contribution

It provides detailed EOS models and superfluidity gap constraints that match observational data, advancing understanding of dense matter in neutron star cores.

Findings

Certain superfluidity gap models are inconsistent with data across all EOS.

Observations can constrain core superfluidity and the symmetry energy slope L.

Models reproduce the neutrino luminosity and heat capacity inferred from observations.

Abstract

Recent observational data on transiently-accreting neutron stars has unequivocally shown fast-cooling sources, such as in the case of neutron star MXB 1659-29. Previous calculations have estimated its total neutrino luminosity and heat capacity, as well as suggested that direct Urca reactions take place in $1 \%$ of the volume of the core. In this paper, we reproduce the inferred luminosity of this source with detailed models of equations of state (EOS) and nuclear pairing gaps. We show that three superfluidity gap models are inconsistent with data for all EOS and another three are disfavoured because of fine tuning arguments. We also calculate the total heat capacity for all constructed stars and show that independent observations of mass and luminosity could set constraints on the core superfluidity of a source as well as the density slope of the symmetry energy, L. This is an important step towards defining a universal equation of state for neutron stars and therefore, towards a better understanding of the phase diagram of asymmetric matter at high densities.