Sensitivity of Neutron Star Observables to Transition Density in Hybrid Equation-of-State Models

Sensitivity of Neutron Star Observables to Transition Density in Hybrid Equation-of-State Models
N. K. Patra, Sk Md Adil Imam, Kai Zhou
arXiv:2604.11046v1 Announce Type: cross
Abstract: We investigate how the transition density (rho_{tr}) affects hybrid constructions of the neutron-star equation of state (EoS) in which a nucleonic description at low densities is matched to a model-agnostic high-density extension based on a speed-of-sound parametrization. Using four representative nucleonic models–Taylor expansion, (frac{n}{3}) expansion, Skyrme, and relativistic mean-field–built from identical nuclear matter parameters, we isolate the impact of the low-density EoS and the transition density on neutron star observables. We find that, within the present smooth-matching prescription, neutron star properties such as radii and tidal deformabilities retain significant sensitivity to the choice of low-density EoS for commonly adopted transition densities around (rho_{tr} approx 2rho_0), even when the same high-density parametrization is employed. This residual dependence arises from differences in the matching conditions at (rho_{tr}), which propagate into the high-density extension, so different low-density inputs lead to different effective high-density EoSs. These findings are robust across two distinct speed-of-sound parametrizations. Quantitatively, the model spread in radius and tidal deformability at $1.4,M_odot$ exceeds the current observational uncertainty by factors of $sim 1.8$ and $sim 1.4$ at $rho_{mathrm{tr}} approx 2rho_0$, whereas these factors reduce to $sim 1.05$ and $sim 0.4$ at $rho_{mathrm{tr}} = rho_0$. Lowering the transition density, therefore, systematically diminishes the spread among models and leads to more consistent predictions. Our results demonstrate that the widely used choice (rho_{tr} approx 2rho_0) does not guarantee model independence in hybrid EoS constructions, and should be treated as an explicit source of systematic uncertainty when inferring dense matter properties from neutron star observations.arXiv:2604.11046v1 Announce Type: cross
Abstract: We investigate how the transition density (rho_{tr}) affects hybrid constructions of the neutron-star equation of state (EoS) in which a nucleonic description at low densities is matched to a model-agnostic high-density extension based on a speed-of-sound parametrization. Using four representative nucleonic models–Taylor expansion, (frac{n}{3}) expansion, Skyrme, and relativistic mean-field–built from identical nuclear matter parameters, we isolate the impact of the low-density EoS and the transition density on neutron star observables. We find that, within the present smooth-matching prescription, neutron star properties such as radii and tidal deformabilities retain significant sensitivity to the choice of low-density EoS for commonly adopted transition densities around (rho_{tr} approx 2rho_0), even when the same high-density parametrization is employed. This residual dependence arises from differences in the matching conditions at (rho_{tr}), which propagate into the high-density extension, so different low-density inputs lead to different effective high-density EoSs. These findings are robust across two distinct speed-of-sound parametrizations. Quantitatively, the model spread in radius and tidal deformability at $1.4,M_odot$ exceeds the current observational uncertainty by factors of $sim 1.8$ and $sim 1.4$ at $rho_{mathrm{tr}} approx 2rho_0$, whereas these factors reduce to $sim 1.05$ and $sim 0.4$ at $rho_{mathrm{tr}} = rho_0$. Lowering the transition density, therefore, systematically diminishes the spread among models and leads to more consistent predictions. Our results demonstrate that the widely used choice (rho_{tr} approx 2rho_0) does not guarantee model independence in hybrid EoS constructions, and should be treated as an explicit source of systematic uncertainty when inferring dense matter properties from neutron star observations.