Continued cooling of the crust in the neutron star low-mass X-ray binary KS 1731-260

TL;DR

This study observes ongoing cooling in the neutron star crust of KS 1731-260, suggesting it has not yet reached thermal equilibrium with the core, providing insights into crustal thermal properties.

Contribution

First long-term observational evidence of continued crustal cooling in KS 1731-260, challenging existing theoretical models of neutron star crust thermal relaxation.

Findings

Crust continues to cool 8 years post-outburst

Cooling follows a simple power-law decay

Crust has not yet equilibrated with the core

Abstract

Some neutron star low-mass X-ray binaries have very long outbursts (lasting several years) which can generate a significant amount of heat in the neutron star crust. After the system has returned to quiescence, the crust then thermally relaxes. This provides a rare opportunity to study the thermal properties of neutron star crusts, putting constraints on the thermal conductivity and hence the structure and composition of the crust. KS 1731-260 is one of only four systems where this crustal cooling has been observed. Here, we present a new Chandra observation of this source approximately 8 years after the end of the last outburst, and 4 years since the last observation. We find that the source has continued to cool, with the cooling curve displaying a simple power-law decay. This suggests that the crust has not fully thermally relaxed yet, and may continue to cool further. A simple power law decay is in contrast to theoretical cooling models of the crust, which predict that the crust should now have cooled to the same temperature as the neutron star core.