Locating the special point of hybrid neutron stars

The special point is a feature unique to models of hybrid neutron stars. It represents location on their mass--radius sequences that insensitive the phase transition density. We consider stars with core deconfined quark matter obeys constant--sound--speed (CSS) equation state model and provide fit formula for coordinates as functions squared sound speed ($c_s^2$) pressure scale ($A$) parameters. Using mass proxy maximum we derive limits CSS parameters based recent NICER constraint radius pulsar PSR J0740+6620, $0.36 < {c^2_s}_{\rm min} 0.43$ $80<A [{\rm MeV/fm}^3]<160$. upper limit depends $c_s^2$ so choosing $c^2_{s,max} = 0.6$ results in $M_{\rm max}<2.7~M_\odot$, within range GW190814.