$K^{-}$-Driven Direct Urca Cooling in Rotating Neutron Stars: A Bayesian Study

Abstract

We investigate the onset of antikaon ($K^{-}$) condensation and its implications for the equation of state (EoS) and cooling of neutron stars (NSs) within density-dependent relativistic mean-field parametrisations DD2 and MPE. Treating the antikaon - nucleon optical potential ($U_K$) as a free parameter in the range $[-180,-60]$ MeV, we constrain it using Bayesian inference with NICER mass-radius observations of PSR J0030+0451 and PSR J0740+6620. The inferred posterior distributions favour strongly attractive in-medium $K^{-}$ interactions, while their broad widths indicate only weak constraints on $U_K$ by astrophysical observations. More attractive values of $U_K$ lead to an earlier onset of $K^{-}$ condensation, enhanced softening of the EoS, and lower Direct Urca (DU) threshold densities. The condensation threshold is systematically lower in DD2 than in MPE, while finite entropy further promotes the onset of rapid cooling. The $K^{-}$-induced enhancement of the proton fraction ($y_p$) substantially affects a larger volume of the stellar core, i.e. capable of sustaining rapid DU cooling. We further show that rapid rotation suppresses DU cooling by reducing the central density and $y_p$, thereby shrinking the DU-active core. This suppression is more pronounced for MPE than for DD2. Our results demonstrate that $K^{-}$ condensation, finite entropy, and rotation jointly exert a strong influence on the conditions for rapid neutrino cooling in NSs.

9 pages, 7 figures