In quasi-persistent neutron star transients , long outbursts cause the neutron star crust to be heated out of thermal equilibrium with the rest of the star .
During quiescence , the crust then cools back down .
Such crustal cooling has been observed in two quasi-persistent sources : KS 1731 - 260 and MXB 1659 - 29 .
Here we present an additional Chandra observation of MXB 1659 - 29 in quiescence , which extends the baseline of monitoring to 6.6 yr after the end of the outburst .
This new observation strongly suggests that the crust has thermally relaxed , with the temperature remaining consistent over 1000 days .
Fitting the temperature cooling curve with an exponential plus constant model we determine an e -folding timescale of 465 \pm 25 days , with the crust cooling to a constant surface temperature of kT _ { eff } ^ { \infty } = 54 \pm 2 eV ( assuming D = 10 kpc ) .
From this , we infer a core temperature in the range ( 3.5 - 8.3 ) \times 10 ^ { 7 } K ( assuming D = 10 kpc ) , with the uncertainty due to the surface composition .
Importantly , we tested two neutron star atmosphere models as well as a blackbody model , and found that the thermal relaxation time of the crust is independent of the chosen model and the assumed distance .