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The heating and cooling of transiently accreting neutron stars provides apowerful probe of the structure and composition of their crust. Observations ofsuperbursts and crust cooling of accretion-heated neutron stars require moreheat release than is accounted for in current models. Obtaining firmconstraints on the depth and magnitude of this extra heat is challenging andtherefore its origin remains uncertain. We report on Swift and XMM-Newtonobservations of the transient neutron star low-mass X-ray binary XTE J1709-267,which were made in 2012 September-October when it transitioned to quiescenceafter a ~10-week long accretion outburst. The source is detected withXMM-Newton at a 0.5-10 keV luminosity of Lx~2E34 (D/8.5 kpc)^2 erg/s. The X-rayspectrum consists of a thermal component that fits to a neutron star atmospheremodel and a non-thermal emission tail, which each contribute ~50% to the totalemission. The neutron star temperature decreases from ~158 to ~152 eV duringthe ~8-hour long observation. This can be interpreted as cooling of a crustallayer located at a column density of y~5E12 g/cm^2 (~50 m inside the neutronstar), which is just below the ignition depth of superbursts. The required heatgeneration in the layers on top would be ~0.06-0.13 MeV per accreted nucleon.The magnitude and depth rule out electron captures and nuclear fusion reactionsas the heat source, but it may be accounted for by chemical separation of lightand heavy nuclei. Low-level accretion offers an alternative explanation for theobserved variability.

A direct measurement of the heat release in the outer crust of the transiently accreting neutron star XTE J1709-267