I-Love-$⟨c_s^2⟩$: Approximately universal relations for the average neutron star stiffness

Abstract

The accurate observations of neutron stars have deepened our knowledge of both general relativity and the properties of nuclear physics at large densities. Relating observations to the microphysics that govern these stars can sometimes be aided by approximate universal relations. One such relation connects the ratio of the central pressure to the central energy density and the compactness of the star, and it has been found to be insensitive to realistic models for the equation of state to a $\sim 10\%$ level. In this paper, we clarify the meaning of the microscopic quantity appearing in this relation, which is reinterpreted as the average of the speed of sound squared in the interior of a star, $⟨c_s^2⟩$. The physical origin of the quasi-universality of the $⟨c_s^2⟩-C$ relation is then investigated. Making use of post-Minkowskian expansions, we find it to be linked to the Newtonian limit of the structure equations, as well as to the fact that the equations of state that describe NSs are relatively stiff. The same post-Minkowskian approach is also applied to the relations between $⟨c_s^2⟩$, the moment of inertia, and the tidal deformability of a neutron star, arriving at similar conclusions.

17 pages, 12 figures

References in corpus (10)

• GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
• PSR J0030+0451 Mass and Radius from NICER Data and Implications for the Properties of Neutron Star Matter
• A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation
• Tidal Love numbers of neutron stars
• Tests of general relativity from timing the double pulsar
• Timing Measurements of the Relativistic Binary Pulsar PSR B1913+16
• Symmetry Energy I: Semi-Infinite Matter
• Constraints on the Symmetry Energy Using the Mass-Radius Relation of Neutron Stars
• Why I-Love-Q
• Unveiling the universality of I-Love-Q relations

Cited by in corpus (10)

• The interplay of astrophysics and nuclear physics in determining the properties of neutron stars
• Novel Scalings of Neutron Star Properties from Analyzing Dimensionless Tolman--Oppenheimer--Volkoff Equations
• Average speed of sound in neutron stars
• Improved Analytic Love-C Relations for Neutron Stars
• Insights Into Neutron Stars From Gravitational Redshifts and Universal Relations
• Exceeding the conformal limit inside rotating neutron stars: Implications to modified theories of gravity
• Is The Trace Anomaly at its Minimum Value at Neutron Star Centers?
• Bayesian Constraints on the Neutron Star Equation of State with a Smooth Hadron-Quark Crossover
• Universal relation between dipole polarizability of finite nuclei and neutron-star compactness
• Recurring region for neutron-star observables