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

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 the 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 the 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 with the use of the Togashi EoS, other fast cooling processes are needed regardless of the surface composition.

Quiescent luminosities of transiently accreting neutron stars with neutrino heating due to charged pion decay

“…2, we examine the effects of neutrino heating on the quiescent luminosity of accreting neutron stars by comparing with the representative observations. For Togashi EOS, as the proton fraction is low that the direct Urca process is prohibited for any mass [44], it cools slowly even for a 2M ⊙ neutron star (dashed lines in Fig. 2).…”

“…For the equation of state (EOS), we adopt Togashi and BSK24 EOSs which are widely used to simulate various astrophysical phenomena [7,12,[43][44][45]. As the Togashi EOS has the low symmetry energy, the nucleon direct Urca process is prohibited with such an EOS.…”

Effects of Nuclear Equation of State on Type-I X-ray Bursts and Implication for Clocked Burster GS 1826–24

“…Many previous studies with multi-zone Xray-burst models, which cover the only accreted layers, introduce a similar artificial heating parameter Q b to impose the envelope and crust boundary conditions (e.g., [1]). To treat the internal energy of NS consistently, we consider the conventional crustal heating process [14] and slow neutrino cooling processes, including the modified Urca and bremsstrahlung (e.g., see [15]). Such cooling processes effectively increase P ign , because they decrease the overall temperature before the NS becomes very old ( 10 5 yrs).…”

Gravitational wave asteroseismology on cooling neutron stars

“…After the EOS and the neutron star mass are selected, how to calculate the neutron star cooling evolution is basically the same as in Refs. [58,59]. In this study, we simply consider the spherically symmetric neutron stars, whose metric is given by…”

“…We note that these equations are practically integrated with the mass coordinate instead of the radial coordinate r (see Refs. [58][59][60] for the concrete equations), in order to carefully treat the region in the TABLE I: EOS parameters adopted in this study, the maximum mass, Mmax/M , for the cold neutron star constructed with each EOS, and the threshold mass for the onset of the direct Urca, MDU/M . The EOS dependence in the cooling curve and gravitational wave frequency is discussed with TM1e and Togashi EOSs, while the other EOSs are considered just for discussion about the EOS dependence of MDU in Fig.…”