After more than a decade of actively accreting at about a tenth of the Eddington critical mass accretion rate , the neutron-star X-ray transient KS 1731–260 returned to quiescence in early 2001 .
We present a Chandra /ACIS-S observation taken several months after this transition .
We detected the source at an unabsorbed flux of \sim 2 \times 10 ^ { -13 } erg cm ^ { -2 } s ^ { -1 } ( 0.5–10 keV ) .
For a distance of 7 kpc , this results in a 0.5–10 keV luminosity of \sim 1 \times 10 ^ { 33 } erg s ^ { -1 } and a bolometric luminosity approximately twice that .
This quiescent luminosity is very similar to that of the other quiescent neutron star systems .
However , if this luminosity is due to the cooling of the neutron star , this low luminosity may indicate that the source spends at least several hundreds of years in quiescence in between outbursts for the neutron star to cool .
If true , then it might be the first such X-ray transient to be identified and a class of hundreds of similar systems may be present in the Galaxy .
Alternatively , enhanced neutrino cooling could occur in the core of the neutron star which would cool the star more rapidly .
However , in that case the neutron star in KS 1731–260 would be more massive than those in the prototypical neutron star transients ( e.g. , Aql X-1 or 4U 1608–52 ) .