发表机构
Centro Brasileiro de Pesquisas Físicas; Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo; Departamento de Física, Instituto Tecnológico de Aeronáutica（巴西物理研究中心; 圣保罗大学天文、地球物理与大气科学研究所; 航空技术学院物理系）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究通过半径分解和贝叶斯推断，证明大质量中子星半径主要由高声速核心设定，外部低密度层仅提供次主导修正，为解释NICER观测提供了新视角。
AI 中文摘要
中子星内部成分探测器（NICER）最近的观测表明，大质量脉冲星PSR J0740+6620（$2.08\\,M_{\odot}$）的半径与PSR J0030+0451及更新分析所推断的接近$1.4\\,M_{\odot}$恒星半径相当。这种近乎垂直的质量-半径行为难以获得，除非高密度状态方程（EOS）强烈变硬。我们表明，对于大质量脉冲星，主导的径向尺度可以由一个相对论性的、高声速的核心设定，该核心始于接近两倍核饱和密度的区域，而外部低密度层（包括真实外壳和外部核心）仅提供次主导的修正。关键证据来自半径分解：将匹配点以下的低密度分支替换为跨越两倍过渡压力的统一EOS模型，会使$2.08\\,M_{\odot}$核心半径仅改变约160米，约为总半径变化的一半。这种收敛确实是大质量现象：在$1.4\\,M_{\odot}$时，核心半径仍然对分支更敏感，变化约0.5公里。固定比例比较证实，随着恒星质量增加，核心主导性系统性地出现，传统的“半径物理”和“最大质量物理”之间的分离在此失效。我们通过结合解析的Tolman VII轮廓和薄壳匹配、数值Tolman-Oppenheimer-Volkoff积分（使用分段EOS）以及直接网格贝叶斯推断（使用NICER和GW170817约束，并施加因果性和质量支持过滤器）来建立这一图景。数据适度约束了过渡刚度，但二次硬化主要由因果性决定。与恒定声速核心的比较仍无定论：两种描述产生相当的核心主导半径，留下简并性，亚公里半径测量可以打破这种简并性。
英文摘要
Recent observations by the Neutron Star Interior Composition Explorer (NICER) indicate that the massive pulsar PSR J0740+6620 ($2.08\,M_{\odot}$) has a radius comparable to those inferred for stars near $1.4\,M_{\odot}$, as shown by PSR J0030+0451 and updated analyses. Such near-vertical mass-radius behavior is difficult to obtain unless the high-density equation of state (EOS) stiffens strongly. We show that, for massive pulsars, the leading radial scale can be set by a relativistic, high-sound-velocity core beginning near twice the nuclear saturation scale, while the outer low-density layer, comprising the true crust and outer core, supplies only a subdominant correction. The key evidence comes from radius decomposition: replacing the low-density branch below the matching point with unified EOS models spanning a factor of two in transition pressure changes the $2.08\,M_{\odot}$ core radius by only $\sim160$ m, about half the variation of the total radius. This convergence is genuinely a high-mass phenomenon: at $1.4\,M_{\odot}$ the core radius remains more branch-sensitive, varying by about $0.5$ km. Fixed-fraction comparisons confirm that core dominance emerges systematically as the star grows more massive, where the traditional separation between ``radius physics'' and ``maximum-mass physics'' breaks down. We establish this picture by combining analytical Tolman VII profiles and thin-crust matching, numerical Tolman-Oppenheimer-Volkoff integration with a piecewise EOS, and direct-grid Bayesian inference using NICER and GW170817 constraints under causal and mass-support filters. Data modestly constrain the transition stiffness but leave the quadratic stiffening governed mainly by causality. A comparison with a constant-sound-speed core remains inconclusive: both descriptions produce comparable core-dominated radii, leaving a degeneracy that sub-kilometre radius measurements can break.
