Locating the special point of hybrid neutron stars

TL;DR

This paper identifies a unique feature called the special point in hybrid neutron star models, providing formulas and constraints on model parameters based on recent observational data, and estimating the maximum mass of such stars.

Contribution

It introduces a fit formula for the special point in hybrid neutron stars with CSS equations of state and derives parameter limits using recent NICER observations.

Findings

The special point is insensitive to phase transition density.

Parameter constraints are derived from NICER data.

Maximum mass estimates depend on sound speed limits.

Abstract

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 ($c_s^2$) 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, $0.36 < {c^2_s}_{\rm min} < 0.43$ and $80<A [{\rm MeV/fm}^3]<160$. The upper limit for the maximum mass of hybrid stars depends on the upper limit for $c_s^2$ so that choosing $c^2_{s,max} = 0.6$ results in $M_{\rm max}<2.7~M_\odot$, within the mass range of GW190814.