The Trace Anomaly at Neutron Star Centers: Minimum or Not?

While the equation of state (EOS) P(ε) of neutron star (NS) matter has been extensively studied, the EOS-parameter ϕ= P/ε or equivalently the dimensionless trace anomaly $Δ= 1/3 - ϕ$, which quantifies the balance between pressure P and energy density ε, 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 $ϕ$ 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 $ϕ$ 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 $ϕ$ at similar densities, reinforcing its monotonic decrease. These model-independent relations impose strong constraints on the near-center behavior of the EOS-parameter $ϕ$, particularly demonstrating that the mean stiffness (or equivalently $Δ$) reaches a local maximum (minimum) at the center.