Strong Gravity Extruding Peaks in Speed of Sound Profiles of Massive Neutron Stars

Abstract

The speed of sound squared (SSS) $s^2$ in massive neutron stars (NSs) characterizes not only the stiffness of supradense neutron-rich matter within but also equivalently properties of the curved geometry due to the strong-field gravity and matter-geometry coupling. A peaked density or radius profile of $s^2$ has been predicted for massive NSs using various NS Equation of State (EOS) models. However, the nature, cause, location and size of the peak in $s^2$ profiles are still very EOS model dependent. In this work, we investigate systematically $s^2$ profiles in massive NSs in a new approach that is independent of the nuclear EOS model and without any presumption about the NS structure and/or composition. In terms of the small quantities (reduced radius, the energy density and pressure scaled by their central values), we perform double-element perturbative expansions in solving perturbatively the scaled Tolman--Oppenheimer--Volkoff (TOV) equations and analyzing $s^2$ profiles from the Newtonian limit to the general relativistic (GR) case. The GR term in the TOV equations plays a twofold role: it compresses NS matter and modifies the pressure/energy density ratio from small values in Newtonian stars showing no $s^2$ peak to large ones for massive NSs possessing a peak in their $s^2$ profiles, and eventually takes away the peak in extremely compact/massive NSs approaching the causality limit. {In particular, the peaked behavior in $s^2$ is expected to emerge near the center of massive NSs like PSR J0740+6620, while a sharp phase transition is unlikely to occur there.} These features revealed from our analyses are universal as they are intrinsic properties of the GR stellar structure equations independent of the still very uncertain EOS of supradense neutron-rich matter in NSs.