The special point on the hybrid star mass--radius diagram and its multi--messenger implications

TL;DR

This paper identifies a special point on the mass-radius diagram of hybrid neutron stars, showing its implications for multi-messenger astronomy and how radius measurements can indicate quark matter cores.

Contribution

It demonstrates the existence and properties of a special point in hybrid star models, independent of hadronic EoS, linking radius measurements to the presence of quark matter cores.

Findings

The special point's location is largely independent of hadronic EoS.

The difference between maximum and special point mass is limited to 0.19 solar masses.

A NICER measurement could identify hybrid neutron stars with quark cores.

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 $0.19~M_\odot$ 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.