Some neutron star low-mass X-ray binaries have very long outbursts (lastingseveral years) which can generate a significant amount of heat in the neutronstar crust. After the system has returned to quiescence, the crust thenthermally relaxes. This provides a rare opportunity to study the thermalproperties of neutron star crusts, putting constraints on the thermalconductivity and hence the structure and composition of the crust. KS 1731-260is 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 afterthe end of the last outburst, and 4 years since the last observation. We findthat the source has continued to cool, with the cooling curve displaying asimple power-law decay. This suggests that the crust has not fully thermallyrelaxed yet, and may continue to cool further. A simple power law decay is incontrast to theoretical cooling models of the crust, which predict that thecrust should now have cooled to the same temperature as the neutron star core.
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