XMM-Newton Observations of the Neutron Star X-Ray Transient KS 1731-260 in Quiescence

We report on XMM-Newton observations performed on 2001 September 13-14 of the neutron star X-ray transient KS 1731-260 in quiescence. The source was detected at an unabsorbed 0.5-10 keV flux of only (4-8)×10-14 ergs cm-2 s-1, depending on the model used to fit the data, which for a distance of 7 kpc implies a 0.5-10 keV X-ray luminosity of approximately (2-5)×1032 ergs s-1. The 2001 September quiescent flux of KS 1731-260 is lower than that observed during the Chandra observation in 2001 March. In the cooling neutron star model for the quiescent X-ray emission of neutron star X-ray transients, this decrease in the quiescent flux implies that the crust of the neutron star in KS 1731-260 cooled down rapidly between the two epochs, indicating that the crust has a high conductivity. Furthermore, enhanced cooling in the neutron star core is also favored by our results