On the Sound Speed in Neutron Stars

Determining the sound speed $c_s$ in compact stars is an important open question with numerous implications on behaviour of matter at large densities and hence gravitational-wave emission from neutron stars. To this scope, we construct more than $10^7$ equations state (EOSs) continuous build $10^8$ nonrotating stellar models consistent not only nuclear theory perturbative QCD, but also astronomical observations. In way, find that EOSs sub-conformal speeds, i.e. $c^2_s < 1/3$ within stars, are possible principle very unlikely practice, being $0.03\%$ our sample. Hence, it natural to expect > somewhere interior. Using sample, obtain estimates $95\%$ credibility neutron-star radii for representative $1.4$ $2.0$ solar masses, $R_{1.4}=12.42^{+0.52}_{-0.99}\,{\rm km}$, $R_{2.0}=12.12^{+1.11}_{-1.23}\,{\rm binary tidal deformability GW170817 event, $\tilde\Lambda_{1.186}=485^{+225}_{-211}$. Interestingly, lower-bounds good agreement prediction derived different arguments, namely, threshold mass. Finally, provide simple analytic expressions determine minimum maximum values $\tilde\Lambda$ as a function chirp