Is The Trace Anomaly at its Minimum Value at Neutron Star Centers?

Abstract

While the equation of state (EOS) $P(\epsilon )$ of neutron star (NS) matter has been extensively studied, the EOS-parameter $\varphi =P/\epsilon$ or equivalently the dimensionless trace anomaly $\Delta =1/3-\varphi$, which quantifies the balance between pressure $P$ and energy density $\epsilon$, remains far less explored, especially in NS cores. Its bounds and density profile carry crucial information about the nature of superdense matter. Physically, the EOS-parameter $\varphi$ represents the mean stiffness of matter accumulated from the stellar surface up to a given density. Based on the intrinsic structure of the Tolman--Oppenheimer--Volkoff equations, we show that $\varphi$ decreases monotonically outward from the NS center, independent of any specific input NS EOS model. Furthermore, observational evidence of a peak in the speed-of-sound squared (SSS) density-profile near the center effectively rules out a valley and a subsequent peak in the radial profile of $\varphi$ at similar densities, reinforcing its monotonic decrease. These model-independent relations impose strong constraints on the near-center behavior of the EOS-parameter $\varphi$, particularly demonstrating that the mean stiffness (or equivalently $\Delta$) reaches a local maximum (minimum) at the center.

Changed the title and added a comment. Phys. Rev. D (2025) in press

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