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

The heating and cooling of transiently accreting neutron stars provides a powerful probe of the structure and composition of their crust. Observations of superbursts and crust cooling of accretion-heated neutron stars require more heat release than is accounted for in current models. Obtaining firm constraints on the depth and magnitude of this extra heat is challenging and therefore its origin remains uncertain. We report on Swift and XMM-Newton observations of the transient neutron star low-mass X-ray binary XTE J1709-267, which were made in 2012 September-October when it transitioned to quiescence after a ~10-week long accretion outburst. The source is detected with XMM-Newton at a 0.5-10 keV luminosity of Lx~2E34 (D/8.5 kpc)^2 erg/s. The X-ray spectrum consists of a thermal component that fits to a neutron star atmosphere model and a non-thermal emission tail, which each contribute ~50% to the total emission. The neutron star temperature decreases from ~158 to ~152 eV during the ~8-hour long observation. This can be interpreted as cooling of a crustal layer located at a column density of y~5E12 g/cm^2 (~50 m inside the neutron star), which is just below the ignition depth of superbursts. The required heat generation 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 reactions as the heat source, but it may be accounted for by chemical separation of light and heavy nuclei. Low-level accretion offers an alternative explanation for the observed variability.