Abstract
We show the existence and investigate the location of the special point (SP) in which hybrid neutron star mass-radius (M-R) curves have to cross each other when they belong to a class of hybrid equation of state (EoS) constructed with generic constant–speed–of–sound (CSS) quark matter models for which the onset deconfinement is varied. We demonstrate that for a three-parameter CSS model the position of the SP in the M-R diagram is largely independent of the choice of the hadronic EoS, but in dependence on the stiffness of the quark matter EoS it spans a region that we identify. We find that the difference between the maximum mass and the SP mass depends on the mass at the onset of deconfinement so that an upper limit of 0.19 M⊙ for this difference is obtained from which a lower limit on the radius of hybrid stars is deduced. Together with a lower limit on the radius of hadronic stars, derived from a class of reasonably soft hadronic EoS including hyperons, we identify a region in the M-R diagram which can be occupied only by hybrid stars. Accordingly, we suggest that a NICER radius measurement on the massive pulsar PSR J0740 + 6620 in the range of 8.6-11.9 km would indicate that this pulsar is a hybrid neutron star with deconfined quark matter in the inner core.
Highlights
A large and comprehensive body of work exists on the equation of state of nuclear matter up to and exceeding the nuclear saturation density that has recently been reviewed in [1]
We have shown the existence and investigated the properties the special point, a characteristic feature shared by a wide range of hybrid neutron star equations of state from the class of two-phase models that allow a systematic variation of a quark matter parameter determining the onset of deconfinement
We have demonstrated that the position of this special point is only marginally depending on the underlying hadronic equation of state
Summary
A large and comprehensive body of work exists on the equation of state of nuclear matter up to and exceeding the nuclear saturation density that has recently been reviewed in [1] Models, such as [2,3,4] are well fitted to existing data on nucleonnucleon interactions, nuclear structure and nuclear matter saturation properties and can be readily used to derive the properties of neutron stars with a hadronic matter core. The state-of-the-art in this regard are classes of effective models, such as the tdBag [24] and the CSS model [25,26], coupled to a separate hadronic equation of state via a Maxwell construction ensuring a first order phase transition Such multi-phase models provide predictions on the M-R relations and the central density of stable neutron stars.
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