A SIGNATURE OF CHEMICAL SEPARATION IN THE COOLING LIGHT CURVES OF TRANSIENTLY ACCRETING NEUTRON STARS

We show that convection driven by chemical separation can significantly affect the cooling light curves of accreting neutron stars after they go into quiescence. We calculate the thermal relaxation of the neutron star ocean and crust including the thermal and compositional fluxes due to convection. After the inward propagating cooling wave reaches the base of the neutron star ocean, the ocean begins to freeze, driving chemical separation. The resulting convection transports heat inward, giving much faster cooling of the surface layers than found assuming the ocean cools passively. The light curves including convection show a rapid drop in temperature weeks after outburst. Identifying this signature in observed cooling curves would constrain the temperature and composition of the ocean as well as offer a real time probe of the freezing of a classical multicomponent plasma.