Symmetry energy effect on rotating neutron stars

¹ Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University Qinhuangdao 066004, China
² School of Physics and Information Engineering, Shanxi Normal University Taiyuan 030031, China
³ School of Science, China University of Petroleum (East China) Qingdao 266580, China
⁴ School of Physics, Nankai University Tianjin 300071, China

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Received: 27 October 2024
Accepted: 1 December 2025

Abstract

We explore the symmetry energy effect on the bulk properties of static neutron stars and rotating millisecond pulsars (MSPs). The unified equations of state (EOSs) are constructed self-consistently within the relativistic mean-field framework from the inner crust to the outer core. To investigate the impact of a unified EOS, which uses the same nuclear interaction for both the crust and core, we compared the results of MSPs with those obtained using non-unified EOSs. These non-unified EOSs match the crust and core EOSs, which have different slopes of symmetry energy. For a given rotational period, we examined how symmetry energy influences the maximum mass, equatorial radius, and deformation from sphericity in MSPs. Our findings indicate that a softer EOS is favored by a higher Keplerian frequency, which corresponds to a smaller L for unified EOSs, but a larger L for the crust in matching EOSs. However, under the slow rotation approximation, there is no significant effect from the symmetry energy slope on the bulk properties of 2 M_⊙ MSPs. In contrast, clear differences are observed for those around and below 1.4 M_⊙.