Thermal evolution of accreting neutron stars

The change in the thermal structure of neutron stars undergoing accretion is computed with a fully general-relativistic evolutionary code. Calculations indicate that initially cold neutron stars are heated up by nuclear burning and/or by the inflow of entropy supplied by the accreted gas; in a relatively short time, they achieve an equilibrium configuration wherein the energy gain due to burning and accretion is balanced exactly by the energy loss through photons and neutrinos. At this stage, the thermal state of the core depends strongly on the neutrino emissivity, while the envelope structure is rather indifferent to this emissivity. The nature of heat flow in the envelope is also studied by using an analytical approximation and the progress of thermal runaways in the accreted envelope is discussed as a function of the charactertistics of the underlying neutron star model.