PROBING THE CRUST OF THE NEUTRON STAR IN EXO 0748–676
X-ray observations of quiescent X-ray binaries have the potential to provide insight into the structure and the composition of neutron stars. EXO 0748–676 had been actively accreting for over 24 yr before its outburst ceased in late 2008. Subsequent X-ray monitoring revealed a gradual decay of the quiescent thermal emission that can be attributed to cooling of the accretion-heated neutron star crust. In this work, we report on new Chandra and Swift observations that extend the quiescent monitoring to ≃5 yr post-outburst. We find that the neutron star temperature remained at ≃117 eV between 2009 and 2011, but had decreased to ≃110 eV in 2013. This suggests that the crust has not fully cooled yet, which is supported by the lower temperature (≃95 eV) measured ≃4 yr prior to the accretion phase in 1980. Comparing the data to thermal evolution simulations reveals that the apparent lack of cooling between 2009 and 2011 could possibly be a signature of convection driven by phase separation of light and heavy nuclei in the outer layers of the neutron star.