Find in Library

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 <inline-formula> <math display="inline"> <semantics> <msub> <mi mathvariant="sans-serif">Λ</mi> <mn>1</mn> </msub> </semantics> </math> </inline-formula>–<inline-formula> <math display="inline"> <semantics> <msub> <mi mathvariant="sans-serif">Λ</mi> <mn>2</mn> </msub> </semantics> </math> </inline-formula> 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 <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>M</mi> <mi>onset</mi> </msub> <mo>≤</mo> <mn>0.8</mn> <mspace width="3.33333pt"></mspace> <msub> <mi>M</mi> <mo>⊙</mo> </msub> </mrow> </semantics> </math> </inline-formula> with an <i>M</i>–<i>R</i> 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 <inline-formula> <math display="inline"> <semantics> <msub> <mi>M</mi> <mi>onset</mi> </msub> </semantics> </math> </inline-formula> = 1.1–1.6 <inline-formula> <math display="inline"> <semantics> <msub> <mi>M</mi> <mo>⊙</mo> </msub> </semantics> </math> </inline-formula> would be favored.