Inferring neutron star properties from GW170817 with universal relations

Abstract

Because all neutron stars share a common equation of state, tidal deformability constraints from the compact binary coalescence GW170817 have implications for the properties of neutron stars in other systems. Using equation-of-state insensitive relations between macroscopic observables like moment of inertia (I), tidal deformability (Λ) and stellar compactness, we derive constraints on these properties as a function of neutron-star mass based on the LIGO-Virgo collaboration's canonical deformability measurement, Λ1.4 = 190 +390 −120 . Specific estimates of Λ, I, dimensionless spin χ, and stellar radius R for a few systems targeted by radio or X-ray studies are extracted from the general constraints. We also infer the canonical neutron-star radius as R1.4 = 10.9 +1.9 −1.5 km at 90% confidence. We further demonstrate how a gravitational-wave measurement of Λ1.4 can be combined with independent measurements of neutron-star radii to tighten constraints on the tidal deformability as a proxy for the equation of state. We find that GW170817 and existing observations of six thermonuclear bursters in low-mass X-ray binaries jointly imply Λ1.4 = 196 +92 −63 at the 90% confidence level.1 A different universal relation has been used elsewhere in conjunction with GW170817 to constrain the maximum mass of nonrotating neutron stars [26]. 2 In the remainder of the paper, quoted error bars refer to symmetric 90% confidence intervals about the median unless otherwise specified. 3 Note added: A Bayesian analysis of this kind-but focused on the stellar radius, rather than the tidal deformability-is presented in Ref.[45], which appeared shortly after completion of this paper.

A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation

“…Studies have utilized joint mass-radius posteriors inferred from X-ray spectral modeling of bursting and quiescent NSs (Steiner et al 2010(Steiner et al , 2013Raithel et al 2017). Moreover, studies have utilized mass and tidal deformability constraints derived from analysis of the NS binary merger event GW170817 (Abbott et al 2018;Annala et al 2018;Lim & Holt 2018;Malik et al 2018;Most et al 2018;Tews et al 2018b;Carson et al 2019;Li & Sedrakian 2019;Montaña et al 2019), and consideration is already being given to combining constraints from electromagnetic and gravitational wave analysis (Kumar & Landry 2019;McNeil Forbes et al 2019;Weih et al 2019). Given a suitable model for the EOS (see, e.g., Read et al 2009;Raithel et al 2016;Lindblom 2018;Tews et al 2018a) there are two approaches to EOS inference: one is to jointly infer the EOS parameters (and central densities) directly from the data (e.g., pulse-profile data); the other is to jointly infer EOS parameters from per-source nuisance-marginalized likelihood functions of exterior-spacetime parameters (e.g., gravitational mass and equatorial radius).…”

Nonparametric inference of neutron star composition, equation of state, and maximum mass with GW170817

“…The derived relations as functions of mass, informed by GW170817, are likely to be of greatest relevance for future observations of both GW events and electromagnetic sources. For example, X-ray timing of pulsars is expected to constrain their masses and radii [56], radio observations of binary pulsars may measure NSs' moments of inertia [50,51,57], and additional GW observations of coalescing binary NSs should produce M -Λ measure-ments consistent with the current constraints [4,58,59]. These measurements will effectively act as a null-test of the hypothesis that all NSs share a single EOS, which is currently difficult to constrain with GW170817 alone (see Ref.…”

Improving the NRTidal model for binary neutron star systems

“…Here, 1 There is also the possibility of extracting EOS information from the spin-spin interaction first entering in the 2PN ψ SS contribution, where the individual terms of ψ SS are proportional to the square of the individual spins, i.e., χ 2 A , χ 2 B , or χ A χ B . Although the maximum NS spin in a BNS is not precisely known, the fastest spinning NS in a BNS system observed to date (PSR J1946+2052 [66]) will only have a dimensionless spin of ∼ 0.02-0.04 at merger [67]. Thus, obtaining EOS information from the spin-spin phase contribution is extremely challenging.…”