Was GW170817 a Canonical Neutron Star Merger? Bayesian Analysis with a Third Family of Compact Stars

Abstract

<jats:p>We investigate the possibility that GW170817 was not the merger of two conventional neutron stars (NS), but involved at least one if not two hybrid stars with a quark matter core that might even belong to a third family of compact stars. To this end, we develop a Bayesian analysis method for selecting the most probable equation of state (EoS) under a set of constraints from compact star physics, which now also include the tidal deformability from GW170817 and the first result for the mass and radius determination for PSR J0030+0451 by the NICER Collaboration. We apply this method for the first time to a two-parameter family of hybrid EoS based on the DD2 model with nucleonic excluded volume for hadronic matter and the color superconducting generalized nlNJL model for quark matter. The model has a variable onset density for deconfinement and can mimic the effects of pasta phases with the possibility of producing a third family of hybrid stars in the mass-radius diagram. The main findings of this study are that: (1) the presence of multiple configurations for a given mass (twins or even triples) corresponds to a set of disconnected lines in the Λ 1 – Λ 2 diagram of tidal deformabilities for binary mergers, so that merger events from the same mass range may result in a probability landscape with different peak positions; (2) the Bayesian analysis with the above observational constraints favors an early onset of the deconfinement transition, at masses of M onset ≤ 0.8 M ⊙ with an M–R relationship that in the range of observed neutron star masses is almost indistinguishable from that of a soft hadronic Akmal, Pandharipande, and Ravenhall (APR) EoS; (3) a few, yet fictitious measurements of the NICER experiment two times more accurate than the present value and a different mass and radius that would change the posterior likelihood so that hybrid EoS with a phase transition onset in the range M onset = 1.1–1.6 M ⊙ would be favored.</jats:p>

Systematic study on the quark-hadron mixed phase in compact stars

“…In almost all combinations of hadronic and quark EOSs along with different values of surface tensions, the quark phase persists in the most massive compact stars, where the fraction ðM QM þ M MP Þ=M ranges from 0 to almost 1. This is in coincidence with the recent studies [127][128][129], which suggest the presence of quark-matter cores inside massive compact stars.…”

Studying the parameters of the extended σ-ω model for neutron star matter

“…. Here we introduced approximations of those estimates by considering a bivariate normal distribution PDF exactly as in [41]. Assuming, those two estimates are equiprobable, since they are mutually exclusive, a likelihood for M -R constraint has been introduced as follows…”

“…A very important aspect shared in these Bayesian studies is the implementation of an unrestricted lower bound for the maximum neutron star mass, a quantity that keeps changing due to the observational updates, therefore influencing the posterior probabilities results [29]. The accompanying analysis [41] to our study here a) focuses on EoS models that show a third branch in the M -R diagram [42][43][44] and, b) uses the same observational constraints and Bayesian methodology used here. In addition, a more detailed overview of the state-of-the-art neutron star Bayesian studies can be found there.…”

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

“…But due to the presently large uncertainties in the simultaneous measurement of masses and radii, e.g., by the NICER experiment [27,28] one cannot yet select a most favorable one among them (cf. [29]). A recent study [30] has found, that EoS from the class of CSS models share the property that all M-R curves, regardless of the hybrid star onset density, must cross a small region in the M-R diagram, the SP.…”