Which Neutron Stars Reach the Stiffening Regime? Multimessenger Constraints on Core Sound Speed and Stellar-Mass Thresholds

TL;DR

This paper uses multimessenger data to infer neutron-star core sound-speed profiles, indicating a high probability of stiffening at intermediate densities and linking this to specific stellar mass thresholds.

Contribution

It provides the first multimessenger inference of neutron-star core stiffening, translating equation-of-state constraints into stellar mass thresholds for the stiffening regime.

Findings

85.4% probability of stiffening at 3.5 times nuclear saturation density in baseline models

Peak stiffening region typically accessed near 2.1 solar masses

Current data mainly constrain entry into the stiffening regime, not the full peak

Abstract

We present a concise multimessenger inference of the neutron-star core sound-speed profile using GW170817 and three \textit{NICER} mass--radius posteriors (PSR J0030$+$0451, PSR J0740$+$6620, and PSR J0437$-$4715). The main result is not only a preference for intermediate-density stiffening within smooth equation-of-state families, but a translation of that inference into the stellar masses that access the relevant density regime. In the baseline smooth peaked family, the posterior probability that $c_s^2 > 1/3$ at $3.5\,n_{\rm sat}$ is $85.4\,\%$, while equal-prior averaging over peaked, monotonic, piecewise, and transition-capable families gives a more conservative $79.0\,\%$. Posterior-resampled exact Tolman--Oppenheimer--Volkoff solutions show that the onset density of the inferred stiffening is typically reached near $1.6\,M_\odot$, whereas the peak region is accessed only near $2.1\,M_\odot$. A J0740-like $2.07\,M_\odot$ pulsar reaches the onset in $91\,\%$ of posterior draws but the peak in only $46\,\%$, showing that current data mainly constrain whether massive stars have entered the stiffening regime rather than traversed its full peak.