The type-I X-ray bursting low mass X-ray binary KS 1731 - 260 was recently detected for the first time in quiescence by Wijnands et al. , following a \tau _ { outburst } \approx 13 yr outburst which ended in Feb 2001 .
We show that the emission area radius for a H atmosphere spectrum ( possibly with a hard power-law component that dominates the emission above 3.5 keV ) is consistent with that observed from other quiescent neutron star transients , R _ { \infty } =23 ^ { +30 } _ { -15 } ( d/8 kpc ) km , and examine possible IR counterparts for KS 1731 - 260 .
Unlike all other known transient neutron stars ( NS ) , the duration of this recent ( and the only observed ) outburst is as long as the thermal diffusion time of the crust .
The large amount of heat deposited by reactions in the crust will have heated the crust to temperatures much higher than the equilibrium core temperature .
As a result , the thermal luminosity currently observed from the neutron star is dominated not by the core , but by the crust .
This scenario implies that the mean outburst recurrence timescale found by Wijnands et al .
( \sim 200 yr ) is a lower limit .
Moreover , because the thermal emission is dominated by the hot crust , the level and the time evolution of quiescent luminosity is determined mostly by the amount of heat deposited in the crust during the most recent outburst ( for which reasonable constraints on the mass accretion rate exist ) , and is only weakly sensitive to the core temperature .
Using estimates of the outburst mass accretion rate , our calculations of the quiescent flux immediately following the end of the outburst agree with the observed quiescent flux to within a factor of a few .
In this paper , we present simulations of the evolution of the quiescent lightcurve for different scenarios of the crust microphysics , and demonstrate that monitoring observations ( with currently flying instruments ) spanning from 1–30 yr can measure the crust cooling timescale and the total amount of heat stored in the crust .
These quantities have not been directly measured for any neutron star .
This makes KS 1731 - 260 a unique laboratory for studying the thermal properties of the crust by monitoring the luminosity over the next few years to decades .