Locating the special point of hybrid neutron stars

The special point is a feature unique to models of hybrid neutron stars. It represents a location on their mass-radius sequences that is insensitive to the phase transition density. We consider hybrid neutron stars with a core of deconfined quark matter that obeys a constant-sound-speed (CSS) equation of state model and provide a fit formula for the coordinates of the special point as functions of the squared sound speed (c_s²) and pressure scale (A) parameters. Using the special point mass as a proxy for the maximum mass of the hybrid stars we derive limits for the CSS model parameters based on the recent NICER constraint on mass and radius of pulsar PSR J0740+6620, 0.36 < C_{s min}² < 0.43 and 80 < A[MeV/fm³] < 160. The upper limit for the maximum mass of hybrid stars depends on the upper limit for c_s² so that choosing c_s,max² = 0.6 results in M_max < 2.7 M_⊙, within the mass range of GW190814.