Trace anomaly in the cores of neutron stars

(1) The data is forthcoming. We will upload the data used in a manuscript after it is accepted for publication. (2) The following is the abstract of the manuscript. While the equation of state (EOS) $P(\varepsilon)$ of neutron star (NS) matter has been extensively studied, the EOS-parameter $\phi = P/\varepsilon$ or equivalently the dimensionless trace anomaly $\Delta = 1/3 - \phi$, which quantifies the balance between pressure $P$ and energy density $\varepsilon$, 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 $\phi$ represents the mean stiffness accumulated from the stellar surface up to a given density. Based on the intrinsic structure of the Tolman--Oppenheimer--Volkoff equations, we show that $\phi$ decreases monotonically outward from the NS center, independent of any specific input EOS model. Furthermore, a peak in the speed-of-sound squared (SSS) profile near the center effectively rules out a valley and a subsequent peak in $\phi$ at similar densities, reinforcing its monotonic decrease. These model-independent relations impose strong constraints on the near-center behavior of the EOS-parameter $\phi$, in particular demonstrating that the mean stiffness (or equivalently $\Delta$) reaches a local maximum (minimum) at the center.