A lower limit on the heat capacity of the neutron star core

TL;DR

This paper establishes a lower limit on neutron star core heat capacity using observational data, ruling out certain exotic core states like the quark CFL phase and guiding future research directions.

Contribution

It introduces a method to constrain neutron star core heat capacity from observational data, providing new limits that challenge some theoretical core models.

Findings

Lower limit below expected for paired nucleons

Rules out quark CFL phase as dominant core state

Future cooling observations will refine constraints

Abstract

We show that observations of the core temperature of transiently-accreting neutron stars combined with observations of an accretion outburst give a lower limit to the neutron star core heat capacity. For the neutron stars in the low mass X-ray binaries KS 1731-260, MXB 1659-29, and XTE J1701-462, we show that the lower limit is a factor of a few below the core heat capacity expected if neutrons and protons in the core are paired, so that electrons provide the dominant contribution to the heat capacity. This limit rules out a core dominated by a quark color-flavor-locked (CFL) phase, which would have a much lower heat capacity. Future observations of or limits on cooling during quiescence will further constrain the core heat capacity.