Causal propagation of signal in strangeon matter

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, $c_s=\sqrt{\mathrm{\partial }P\mathrm{/}\mathrm{\partial }\rho }$, 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., $c_{\mathrm{s}\mathrm{i}\mathrm{g}\mathrm{n}\mathrm{a}\mathrm{l}}<c$, and the signal speed as a function of stellar radius is presented.