Crustal cooling of accretion-heated neutron stars provides insight into the
stellar interior of neutron stars. The neutron star X-ray transient,
KS~1731−260, was in outburst for 12.5 years before returning to quiescence in
2001. We have monitored the cooling of this source since then through {\it
Chandra} and {\it XMM-Newton} observations. Here, we present a 150 ks {\it
Chandra} observation of KS~1731−260 taken in August 2015, about 14.5 years
into quiescence, and 6 years after the previous observation. We find that the
neutron star surface temperature is consistent with the previous observation,
suggesting that crustal cooling has likely stopped and the crust has reached
thermal equilibrium with the core. Using a theoretical crust thermal evolution
code, we fit the observed cooling curves and constrain the core temperature
(Tc=9.35±0.25×107 K), composition (Qimp=4.4−0.5+2.2)
and level of extra shallow heating required (Qsh=1.36±0.18
MeV/nucleon). We find that the presence of a low thermal conductivity layer, as
expected from nuclear pasta, is not required to fit the cooling curve well, but
cannot be excluded either.Comment: 7 pages, 6 figures, 1 table, accepted to Ap