Rotochemical heating of old neutron stars

As the rotation of a neutron star slows down, its density increases, pushing the matter inside the star out of beta equilibrium. The beta decays restoring this equilibrium dissipate energy, which heats the star, making it emit thermal ultraviolet radiation for much longer than the standard cooling times. This effect might have been observed in the nearest millisecond pulsar, J0437-4715, by Kargaltsev and collaborators. We show how it operates if the nucleons in the star are superfluid. The steady-state chemical imbalances and temperature increase with the energy gaps, making it possible to constrain the energy gaps from observations. In certain cases, strong oscillations of the temperature can occur. We also analyze competing heating mechanisms, such as vortex friction, and suggest observations that might distinguish between them.