Internal heating mechanisms in neutron stars

The cooling mechanisms of a neutron star (hereafter referred to as NS) have the potential to reveal important features of superdense matter. The values of the surface temperatures are known for a good sample of NSs aged <= 10(6) years and, with some exceptions, can be explained through standard cooling mechanisms (neutrinos and photons emissions without internal heating), as confirmed in our study. However, for older objects the surface temperatures are in some cases higher than expected, and it is necessary to consider some kind of internal heating to explain these results. With this objective, we revisit in this paper the kinetic mechanisms of heating in NSs considering fermionic dark matter (DM) heating, rotochemical heating and magnetic field decay. Our results indicate that NSs older than similar to 10(6) years, such as some "Black Widows" (BW - a subset of binary systems in which only the upper bounds of surface temperatures are known) and old pulsars, in contrast to younger NSs, exhibit much higher surface temperatures than the values predicted by these three heating mechanisms. Furthermore, by restricting the DM heating parameters to the current values that were fitted and/or measured for the local DM density, masses and NS radii, the models studied here also do not reproduce the upper limits of the temperatures from the surface of BWs or the actual temperatures of other ancient pulsars. We conclude that if the upper limits for BWs are close to real temperatures, dark heating will not represent a convincing explanation of these results, indicating that rotochemical mechanisms may be favored.