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Constraining of Nuclear Matter Equations of State With Rotating Neutron Stars

Hyukjin Kwon, Kazuyuki Sekizawa

TL;DR

This study constrains nuclear matter EoSs by modeling rapidly rotating neutron stars with the Komatsu-Eriguchi-Hachisu (KEH) method using Gogny finite-range interactions. It demonstrates that rotation systematically stiffens the mass-radius relation, increases the maximum mass and equatorial radius, and reduces central density, with the effect growing at higher spin frequencies. Among seven Gogny EoSs, six predict nonphysical negative symmetry energy at high density (PN matter), while D1M* remains well-behaved, underscoring the importance of EoS selection when interpreting observations. The results highlight that rotational effects cannot be neglected in EoS constraints from neutron-star data and pave the way for future work linking the symmetry-energy slope $L$ to rotation-induced radius changes.

Abstract

Neutron stars can be regarded as natural laboratories that enable us to investigate nuclear matter properties under extreme conditions that are otherwise impossible to access in terrestrial experiments. Astrophysical observations of neutron stars provide invaluable information on existing nuclear interaction models and equations of state (EoSs) at various densities. Most studies of neutron star structure employ the Tolman-Oppenheimer-Volkoff (TOV) equation which describes spherically symmetric, non-rotating stars in hydrostatic equilibrium. However, since neutron stars rotate fast, they could experience significant centrifugal deformation, and axially-symmetric calculations are required for accurate description of internal structure. The Komatsu-Eriguchi-Hachisu (KEH) method is well known for modeling rapidly-rotating compact objects in a fully general relativistic manner. In this contribution, we report results of KEH calculations for rapidly-rotating neutron stars using EoSs based on Gogny-type finite-range effective nucleon-nucleon interactions. Our results show that the mass-radius relation systematically changes with increasing angular velocity, highlighting the importance of including rotational effects when confronting theoretical EoSs with observational data.

Constraining of Nuclear Matter Equations of State With Rotating Neutron Stars

TL;DR

This study constrains nuclear matter EoSs by modeling rapidly rotating neutron stars with the Komatsu-Eriguchi-Hachisu (KEH) method using Gogny finite-range interactions. It demonstrates that rotation systematically stiffens the mass-radius relation, increases the maximum mass and equatorial radius, and reduces central density, with the effect growing at higher spin frequencies. Among seven Gogny EoSs, six predict nonphysical negative symmetry energy at high density (PN matter), while D1M* remains well-behaved, underscoring the importance of EoS selection when interpreting observations. The results highlight that rotational effects cannot be neglected in EoS constraints from neutron-star data and pave the way for future work linking the symmetry-energy slope to rotation-induced radius changes.

Abstract

Neutron stars can be regarded as natural laboratories that enable us to investigate nuclear matter properties under extreme conditions that are otherwise impossible to access in terrestrial experiments. Astrophysical observations of neutron stars provide invaluable information on existing nuclear interaction models and equations of state (EoSs) at various densities. Most studies of neutron star structure employ the Tolman-Oppenheimer-Volkoff (TOV) equation which describes spherically symmetric, non-rotating stars in hydrostatic equilibrium. However, since neutron stars rotate fast, they could experience significant centrifugal deformation, and axially-symmetric calculations are required for accurate description of internal structure. The Komatsu-Eriguchi-Hachisu (KEH) method is well known for modeling rapidly-rotating compact objects in a fully general relativistic manner. In this contribution, we report results of KEH calculations for rapidly-rotating neutron stars using EoSs based on Gogny-type finite-range effective nucleon-nucleon interactions. Our results show that the mass-radius relation systematically changes with increasing angular velocity, highlighting the importance of including rotational effects when confronting theoretical EoSs with observational data.

Paper Structure

This paper contains 9 sections, 14 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Equation of State (EoS)
  3. Effective nucleon-nucleon interactions
  4. Nuclear Matter EoS
  5. Models of Neutron Star Structure
  6. Structure of Static Neutron Stars
  7. Structure of Rotating Neutron Stars
  8. Results
  9. Summary and Conclusions

Figures (2)

  • Figure 1: Pressure as a function of baryon (nucleon) number density for $npe\mu$ matter calculated with seven Gogny-type effective nucleon-nucleon interactions examined.
  • Figure 2: (a--b, d--e) Density distributions of $1.4 \ M_\odot$ neutron stars calculated with the Gogny D1M* equation of state (EoS) at rotational frequencies of 200, 400, 600, and 800 Hz. The color scale represents the mass density in units of $10^{14} \ \text{g/cm}^3$. As the rotational frequency increases, the stellar shape becomes more and more oblate due to centrifugal deformation. (c) Mass-radius relations of neutron stars obtained using the Gogny D1M* EoS at different rotation frequencies. The color of each curve denotes the ratio of the polar to equatorial radii, $R_{\text{ratio}}=R_\text{p}/R_\text{e}$. Faster rotation leads to an increase in both the maximum mass and the equatorial radius.