Combining the GW observations of merging systems of binary neutron stars and quasi-universal relations, we set constraints on the maximum mass that can be attained by nonrotating stellar models of neutron stars. More specifically, exploiting the recent observation of the GW event GW 170817 and drawing from basic arguments on kilonova modeling of GRB 170817A, together with the quasi-universal relation between the maximum mass of nonrotating stellar models M TOV and the maximum mass supported through uniform rotation M max = 1.20 +0.02 −0.05 M TOV we set limits for the maximum mass to be 2.01 +0.04 −0.04 ≤ M TOV /M 2.16 +0.17 −0.15 , where the lower limit in this range comes from pulsar observations. Our estimate, which follows a very simple line of arguments and does not rely on the modeling of the electromagnetic signal in terms of numerical simulations, can be further refined as new detections become available. We briefly discuss the impact that our conclusions have on the equation of state of nuclear matter.
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
The recent detection of GW190814 featured the merger of a binary with a primary having a mass of ∼23 M⊙ and a secondary with a mass of ∼2.6 M⊙. While the primary was most likely a black hole, the secondary could be interpreted as either the lightest black hole or the most massive neutron star ever observed, but also as the indication of a novel class of exotic compact objects. We here argue that although the secondary in GW190814 is most likely a black hole at merger, it needs not be an ab-initio black hole nor an exotic object. Rather, based on our current understanding of the nuclear-matter equation of state, it can be a rapidly rotating neutron star that collapsed to a rotating black hole at some point before merger. Using universal relations connecting the masses and spins of uniformly rotating neutron stars, we estimate the spin, $0.49_{-0.05}^{+0.08} \lesssim \chi \lesssim 0.68_{-0.05}^{+0.11}$, of the secondary – a quantity not constrained so far by the detection – and a novel strict lower bound on the maximum mass, $M_{_{\mathrm{TOV}}}> 2.08^{+0.04}_{-0.04}\, \, M_{\odot }$ and an optimal bound of $M_{_{\mathrm{TOV}}}> 2.15^{+0.04}_{-0.04}\, \, M_{\odot }$, of nonrotating neutron stars, consistent with recent observations of a very massive pulsar. The new lower bound also remains valid even in the less likely scenario in which the secondary neutron star never collapsed to a black hole.
With the first detection of gravitational waves from a binary system of neutron stars (BNS), GW170817, a new window was opened to study the properties of matter at and above nuclear saturation density. Reaching densities a few times that of nuclear matter and temperatures up to 100 MeV, such mergers also represent potential sites for a phase transition (PT) from confined hadronic matter to deconfined quark matter. While the lack of a post-merger signal in GW170817 has prevented us from assessing experimentally this scenario, two theoretical studies have explored the post-merger gravitational-wave signatures of PTs in BNS mergers. We here extend and complete the picture by presenting a novel signature of the occurrence of a PT. More specifically, using fully general-relativistic hydrodynamic simulations and employing a suitably constructed equation of state that includes a PT, we present the occurrence of a "delayed PT", i.e., a PT that develops only some time after the merger and produces a metastable object with a quark-matter core, i.e., a hypermassive hybrid star. Because in this scenario, the post-merger signal exhibits two distinct fundamental gravitational-wave frequencies -before and after the PT -the associated signature promises to be the strongest and cleanest among those considered so far, and one of the best signatures of the production of quark matter in the present Universe.
The stability properties of rotating relativistic stars against prompt gravitational collapse to a black hole are rather well understood for uniformly rotating models. This is not the case for differentially rotating neutron stars, which are expected to be produced in catastrophic events such as the merger of binary system of neutron stars or the collapse of a massive stellar core. We consider sequences of differentially rotating equilibrium models using the j-constant law and by combining them with their dynamical evolution, we show that a sufficient stability criterion for differentially rotating neutron stars exists similar to the one of their uniformly rotating counterparts. Namely: along a sequence of constant angular momentum, a dynamical instability sets in for central rest-mass densities slightly below the one of the equilibrium solution at the turning point. In addition, following Breu & Rezzolla (2016), we show that "quasi-universal" relations can be found when calculating the turning-point mass. In turn, this allows us to compute the maximum mass allowed by differential rotation, M max,dr , in terms of the maximum mass of the nonrotating configuration, M TOV , finding that M max,dr (1.54 ± 0.05) M TOV for all the equations of state we have considered.
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