Gapless neutron superfluidity in the crust of the accreting neutron stars KS 1731-260 and MXB 1659-29

TL;DR

This study explores how gapless neutron superfluidity, caused by superflows and vortex pinning, can explain the late-time cooling behavior of accreting neutron stars MXB 1659-29 and KS 1731-260, using advanced simulations.

Contribution

It introduces the concept of gapless superfluidity due to superflows in neutron star crusts and demonstrates its effectiveness in modeling observed cooling data.

Findings

Gapless superfluidity models fit observational data better.

Superflows can induce a new gapless superfluid phase.

Predictions made for future observational tests.

Abstract

The interpretation of the thermal evolution of the transiently accreting neutron stars MXB 1659-29 and KS 1731-260 after an outburst is challenging, both within the traditional deep-crustal heating paradigm and the thermodynamically consistent approach of Gusakov and Chugunov that accounts for neutron diffusion throughout the crust. All these studies assume that the neutron superfluid in the crust is at rest. However, we have recently shown that a finite superflow could exist and could lead to a new gapless superfluid phase if quantized vortices are pinned. We have revisited the cooling of MXB 1659-29 and KS 1731-260 and we have found that gapless superfluidity could naturally explain their late time cooling. We pursue here our investigation by performing new simulations of the thermal relaxation of the crust of MXB 1659-29 and KS 1731-260 within a Markov Chain Monte Carlo method accounting for neutron diffusion and allowing for gapless superfluidity. We have varied the global structure of the neutron star, the composition of the heat-blanketing envelope, and the mass accretion rate. In all cases, observations are best fitted by models with gapless superfluidity. Finally, we make predictions that could be tested by future observations.