Causal propagation of signal in strangeon matter

TL;DR

This paper investigates the actual signal propagation speed in strangeon matter, ensuring it respects causality, and provides a detailed analysis of how this speed varies within strangeon stars.

Contribution

It introduces a realistic calculation of signal speed in strangeon matter, addressing superluminal issues and ensuring causality is maintained.

Findings

Signal speed is less than the speed of light, satisfying causality.

The propagation speed varies with stellar radius.

The study clarifies dynamic responses in strangeon stars.

Abstract

The state equation of strangeon matter is very stiff due to the non-relativistic nature of and the repulsing interaction between the particles, and pulsar masses as high as $\sim 3M_\odot$ would be expected. However, an adiabatic sound speed, $c_s=\sqrt{\partial P/\partial \rho}$, is usually superluminal for strangeon matter, and dynamic response of strangeon star (e.g., binary merger) could not be tractable in calculation. We examine signal propagation in strangeon matter, and calculate the propagation speed, $c_{\rm signal}$, in reality. It is found that as the causality condition is satisfied, i.e., $c_{\rm signal}<c$, and the signal speed as a function of stellar radius is presented.