Late time cooling of neutron star transients and the physics of the inner crust

TL;DR

This paper investigates the late-time cooling behavior of neutron star transients, revealing that a low thermal conductivity layer, possibly nuclear pasta, affects the thermal relaxation process and the formation of a normal neutron layer near the crust-core boundary.

Contribution

It introduces a model explaining prolonged crust cooling in MXB 1659-29 by considering a low thermal conductivity layer, challenging previous predictions of faster thermal equilibrium.

Findings

Late-time cooling suggests a low thermal conductivity layer near the crust-core boundary.

The observed cooling is consistent with a normal neutron layer maintaining a temperature gradient.

Thermal transport properties significantly influence neutron star crust cooling timescales.

Abstract

An accretion outburst onto a neutron star transient heats the neutron star's crust out of thermal equilibrium with the core. After the outburst the crust thermally relaxes toward equilibrium with the neutron star core and the surface thermal emission powers the quiescent X-ray light curve. Crust cooling models predict that thermal equilibrium of the crust will be established $\approx 1000 \, \mathrm{d}$ into quiescence. Recent observations of the cooling neutron star transient MXB 1659-29, however, suggest that the crust did not reach thermal equilibrium with the core on the predicted timescale and continued to cool after $\approx 2500 \, \mathrm{d}$ into quiescence. Because the quiescent light curve reveals successively deeper layers of the crust, the observed late time cooling of MXB 1659-29 depends on the thermal transport in the inner crust. In particular, the observed late time cooling is consistent with a low thermal conductivity layer near the depth predicted for nuclear pasta that maintains a temperature gradient between the neutron star's inner crust and core for thousands of days into quiescence. As a result, the temperature near the crust-core boundary remains above the critical temperature for neutron superfluidity and a layer of normal neutrons forms in the inner crust. We find that the late time cooling of MXB 1659-29 is consistent with heat release from a normal neutron layer near the crust-core boundary with a long thermal time.