New Constraints on Radii and Tidal Deformabilities of Neutron Stars from GW170817

Abstract

We explore in a parameterized manner a very large range of physically plausible equations of state (EOSs) for compact stars for matter that is either purely hadronic or that exhibits a phase transition. In particular, we produce two classes of EOSs with and without phase transitions, each containing one million EOSs. We then impose constraints on the maximum mass (M<2.16 M_{⊙}) and on the dimensionless tidal deformability (Λ[over ˜]<800) deduced from GW170817, together with recent suggestions of lower limits on Λ[over ˜]. Exploiting more than 10^{9} equilibrium models for each class of EOSs, we produce distribution functions of all the stellar properties and determine, among other quantities, the radius that is statistically most probable for any value of the stellar mass. In this way, we deduce that the radius of a purely hadronic neutron star with a representative mass of 1.4 M_{⊙} is constrained to be 12.00<R_{1.4}/km<13.45 at a 2σ confidence level, with a most likely value of R[over ¯]_{1.4}=12.39 km; similarly, the smallest dimensionless tidal deformability is Λ[over ˜]_{1.4}>375, again at a 2σ level. On the other hand, because EOSs with a phase transition allow for very compact stars on the so-called "twin-star" branch, small radii are possible with such EOSs although not probable, i.e., 8.53<R_{1.4}/km<13.74 and R[over ¯]_{1.4}=13.06 km at a 2σ level, with Λ[over ˜]_{1.4}>35.5 at a 3σ level. Finally, since these EOSs exhibit upper limits on Λ[over ˜], the detection of a binary with a total mass of 3.4 M_{⊙} and Λ[over ˜]_{1.7}>461 can rule out twin-star solutions.

Empirical constraints on the high-density equation of state from multimessenger observables

“…The obtained results are also in agreement with [52], where the maximum value R 1.4M⊙ = 13.6 km and the minimum value Λ 1.4M⊙ = 120 were reported, using a generic family of EoS that interpolate between chiral effective field theory results at low densities and perturbative QCD at high densities. Furthermore, our results are compatible with [53] (an extra condition on the allowed maximum NS mass was imposed, M max < 2.16M ⊙ , though), in which a mean value of R 1.4M⊙ = 12.39 km and a 2σ confidence of 12.00 < R 1.4M⊙ /km < 13.45 were determined using a piecewise polytrope parametrization of the EoS, which took into account nuclear matter calculations of the outer crust, near saturation densities, and perturbative QCD. Interestingly, the minimum value obtained for Λ 1.4M⊙ is 243.53 and, furthermore, only a very small percentage of the EoS failed to reproduce Λ 1.4M⊙ < 580 (the maximum value reached for Λ 1.4M⊙ was 651.85).…”

On the deconfinement phase transition in neutron-star mergers

“…Finally, we note that the study of all these key thermodynamical quantities is useful to validate whether the physical conditions produced in neutron-star mergers are indeed similar to the those generated in other physical scenarios, such as supernova explosions or relativistic heavyion collisions [40]. More specifically, we have found this analogy to hold reasonably well when comparing our temperatures and entropies with the conditions encountered in the matter ejected in supernova explosions or in heavyion collisions at low energies, such as the ones to be produced in FAIR and NICA.…”

Relativistic hybrid stars with sequential first-order phase transitions and heavy-baryon envelopes

“…One possibility is a deconfinement phase transition from bound hadronic states to liberated quark states. The effect of a strong first-order phase transition on tidal deformabilities of the neutron star in the context of GW170817 event has been explored by a number of * jiajieli@itp.uni-frankfurt.de † sedrakian@fias.uni-frankfurt.de ‡ alford@physics.wustl.edu groups [5][6][7][8][9][10][11][12][13][14][15] showing that the hadron-quark phase transition at low enough density, ρ 2-3ρ 0 , where ρ 0 is the nuclear saturation density, relaxes the tension between the inferred tidal deformabilities and those predicted by purely hadronic models without a phase transition. Of particular interest in this context is the emergence of twin stars, where purely nucleonic and hybrid stars have the same masses, but different radii [6].…”