Equation-of-state-insensitive measure of neutron star stiffness

Abstract

Universal relations (i.e., insensitive to the equation of state) between macroscopic properties of neutron stars have proven useful for a variety of applications -- from providing a direct means to extract observables from data to breaking degeneracies that hinder tests of general relativity. Similarly, equation-of-state-insensitive relations directly connecting macroscopic and microscopic properties of neutron stars can potentially provide a clean window into the behavior of nuclear matter. In this work, we uncover a tight correlation between certain macroscopic properties of a neutron star - its compactness $C$, moment of inertia $\bar{I}$ and tidal deformability $\bar{\Lambda }$ - and the ratio ${\alpha }_c\equiv p_c/{\epsilon }_c$ of central pressure to central energy density, which can be interpreted as a mean notion of the stiffness of nuclear matter inside that object. We describe interesting properties of this stiffness measure, quantify the (approximate) universality of the ${\alpha }_c-C/\bar{I}/\bar{\Lambda }$ relations, and explore its consequences in the face of recent and future neutron star observations.

arXiv comment10 pages, 9 figures; extended and revised to match published version

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• Systematic errors due to quasi-universal relations in binary neutron stars and their correction for unbiased model selection