Characterizing the breakdown of quasi-universality in the post-merger gravitational waves from binary neutron star mergers

Abstract

The post-merger gravitational wave (GW) emission from a binary neutron star merger is expected to provide exciting new constraints on the dense-matter equation of state (EoS). Such constraints rely, by and large, on the existence of quasi-universal relations, which relate the peak frequencies of the postmerger GW spectrum to properties of the neutron star structure in a model-independent way. In this work, we report on violations of existing quasi-universal relations between the peak spectral frequency, f 2 , and the stellar radius, for EoSs models with backwards-bending slopes in their mass-radius relations (such that the radius increases at high masses). The violations are extreme, with variations in f 2 of up to ∼ 600 Hz between EoSs that predict the same radius for a 1.4 M neutron star, but that have significantly different radii at higher masses. Quasi-universality can be restored by adding in a second parameter to the fitting formulae that depends on the slope of the mass-radius curve. We further find strong evidence that quasi-universality is never broken for the radii of very massive stars (with masses 2 M ). Both statements imply that f 2 is mainly sensitive to the high-density EoS. Combined with observations of the binary neutron star inspiral, these generalized quasi-universal relations can be used to simultaneously infer the characteristic radius and slope of the neutron star mass-radius relation.

Kilohertz Gravitational Waves From Binary Neutron Star Mergers: Numerical-relativity Informed Postmerger Model

“…An example illustrating this fact is the breaking of the quasiuniversal relations claimed in Ref. [123]. The latter refers to relations of type f 2 (R) that are different from those employed here.…”

“…In Appendix F, we verified that those f 2 (R) relations are broken also by some data of the CoRe database. The additional term proposed in Raithel and Most [123] does not fix the breaking of some CoRe data with softening effects at high densities. However, we verified that the use of mass-rescaled quantities, i.e.…”

“…In this appendix, we discuss the quasiuniversal relations between the PM peak frequency f 2 and the NS radius at fiducial values in light of the results of [27,123]. In particular, we calibrate the relations f 2 (R 1.4 ) and f 2 (R 1.8 ) including the CoRe data [16,57,62,73,85], where R 1.4 (R 1.8 ) is the radius of a 1.4 M (1.8 M ) NS computed from the TOV equations.…”

“…Following [123], we employ a quadratic relation for the calibration of the PM peak as function of the NS radius, including linear corrections in the ratio R 1.4 /R 1.8 , i.e.…”

Binary neutron star mergers as a probe of quark-hadron crossover equations of state

“…A recent work by Ref. [83] has also found a diminished f peak value from their stiffened EOS models. Since the ppQHCs's Λ 1.35 are similar to that of the SLy, the small f peak of the ppQHCs will violate the universal relation in terms of Λ and show a shift below the universality relation contrary to the shift that appeared above in the phase transition involved models (See, Fig.…”