Possibility of rapid neutron star cooling with the realistic equation of state

TL;DR

This study investigates neutron star cooling using realistic equations of state, finding that the Togashi EoS predicts slow cooling due to the suppression of the direct Urca process, implying other fast cooling mechanisms are necessary to match observations.

Contribution

It demonstrates that the Togashi EoS, with realistic nuclear potentials, results in slow neutron star cooling by forbidding the direct Urca process, highlighting the importance of alternative fast cooling processes.

Findings

Togashi EoS predicts slow cooling due to low symmetry energy.

Direct Urca process is forbidden with Togashi EoS.

Other fast cooling processes are needed to match observations.

Abstract

Whether fast cooling processes occur or not is crucial for the thermal evolution of neutron stars. In particular, the threshold of the direct Urca process, which is one of the fast cooling processes, is determined by the interior proton fraction $Y_p$, or the nuclear symmetry energy. Since recent observations indicate the small radius of neutron stars, a low value is preferred for the symmetry energy. In this study, simulations of neutron star cooling are performed adopting three models for equation of state (EoS): Togashi, Shen, and LS220 EoSs. The Togashi EoS has been recently constructed with realistic nuclear potentials under finite temperature, and found to account for the small radius of neutron stars. As a result, we find that, since the direct Urca process is forbidden, the neutron star cooling is slow with use of the Togashi EoS. This is because symmetry energy of Togashi EoS is lower than those of other EoSs. Hence, in order to account for observed age and surface temperature of isolated neutron stars (INS) with use of the Togashi EoS, other fast cooling processes are needed regardless of the surface composition.