A Chandra observation of the long-duration X-ray transient KS 1731-260 in quiescence: Too cold a neutron star?

After more than a decade of actively accreting at about a tenth of the Eddi ngton critical mass accretion rate, the neutron star X-ray transient KS 173 1-260 returned to quiescence in early 2001. We present a Chandra/ Advanced CCD Imaging Spectrometer observation taken several months after this transi tion. We detected the source at an unabsorbed flux of similar to2 x 10(-13) ergs cm(-2) s(-1) (0.5-10 keV). For a distance of 7 kpc, this results in a 0.5-10 keV luminosity of similar to1 x 10(33) ergs s(-1) and a bolometric luminosity approximately twice that. This quiescent luminosity is very simi lar to that of the other quiescent neutron star systems. However, if this l uminosity is due to the cooling of the neutron star, this low luminosity ma y indicate that the source spends at least several hundreds of years in qui escence 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 h undreds of similar systems may be present in the Galaxy. Alternatively, enh anced neutrino cooling could occur in the core of the neutron star that wou ld cool the star more rapidly. However, in that case, the neutron star in K S 1731-260 would be more massive than those in the prototypical neutron sta r transients (e.g., Aquila X-1 or 4U 1608-52).