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 (cs2) 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 < Cs min2 < 0.43 and 80 < A[MeV/fm3] < 160. The upper limit for the maximum mass of hybrid stars depends on the upper limit for cs2 so that choosing cs,max2 = 0.6 results in Mmax < 2.7 M⊙, within the mass range of GW190814.
Highlights
We will restrict ourselves to the standard Maxwell construction of a phase transition and make use of the novel insights due to the special point property of hybrid neutron star sequences [30–33] in order to determine the impact of the recent observations on the available parameter space of a class of constant–speed–of–sound (CSS) quark–gluon plasma (QGP) models
The accuracy of this approximation can be judged from figures 3 and 4, where the data points were taken from [33] and the black solid lines are obtained using the fit formulas of Eq (5) with the parameters listed in table 1
The special point typically corresponds to a mass slightly below Mmax, no less than Mmax − MSP = 0.2 M [31]
Summary
The recent observation of gravitational waves originating from the inspiral of a binary system of neutron stars has resulted in a significant step forward in the field of nuclear astrophysics. A possible alternative are multi–phase models, predicting a phase transition from purely hadronic neutron stars at low mass to intermediate and high mass hybrid stars with a core presenting exotic states of matter, for instance a quark–gluon plasma (QGP). A number of alternative approaches were proposed [8, 28, 29] which join a soft low-density hadronic EoS with sti↵ QGP models at high densities, fulfilling all observational constraints In this manuscript, we will restrict ourselves to the standard Maxwell construction of a phase transition and make use of the novel insights due to the special point property of hybrid neutron star sequences [30–33] in order to determine the impact of the recent observations on the available parameter space of a class of constant–speed–of–sound (CSS) QGP models [34], which were shown to provide an accurate description of color–superconducting quark matter [35, 36].
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