Speaker
Description
The rapid cooling observed in the Cassiopeia A neutron star (CasA NS) is one of the most stringent tests for neutron-star cooling theory. While Cooper-pair breaking and formation (PBF) neutrino emission is a leading candidate, uncertainties remain regarding the PBF efficiency factor $q$ and the neutron ${}^3$P${}_2$ pairing gap. This work [1] explores in a data-driven manner how the optimized gap shape responds to variations of the PBF emissivity parameter within a fixed cooling setup. We introduce a novel gap parametrization, in which each parameter carries direct physical meaning and controls the gap amplitude, peak location, width, and asymmetry. Using a Fortran-based cooling code and the BSk24 equation of state, we perform parameter-space exploration guided by the CasA NS data. Global optimization is carried out with Optuna's tree-structured Parzen estimator, followed by local refinement using the Nelder-Mead method. The optimized solutions yield physically reasonable gaps with peak amplitudes $\Delta_\text{max}\approx0.5-0.6$ MeV. For $M=1.4M_\odot$, increasing $q$ drives the optimized gap and critical-temperature profiles toward smoother and more localized shapes, improving consistency with the observed trend. Models with $q\gtrsim0.4$ reproduce the decline rate within the $1\sigma$ confidence interval, whereas the baseline case $q\simeq0.19$ lies near the $3\sigma$ level. Our results suggest larger effective PBF emissivities than the baseline estimate. As a next step, we are undertaking Bayesian inference including uncertainties in mass, envelope composition, equation of state, pairing microphysics, and age offset [2]. In this talk, we will show how we can get deeper insight into dense nuclear matter by using precise observational data of neutron star surface temperatures, like CasA NS.
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[1] Y. Nam and K. Sekizawa, Phys. Rev. C 113, 045807 (2026).
[2] K. Sekizawa and Y. Nam, in preparation.
| Category | Theory |
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