Quasiuniversal Properties of Neutron Star Mergers

Abstract

Binary neutron star mergers are studied using nonlinear 3+1 numerical relativity simulations and the analytical effective-one-body (EOB) model. The EOB model predicts quasiuniversal relations between the mass-rescaled gravitational wave frequency and the binding energy at the moment of merger, and certain dimensionless binary tidal coupling constants depending on the stars Love numbers, compactnesses and the binary mass ratio. These relations are quasiuniversal in the sense that, for a given value of the tidal coupling constant, they depend significantly neither on the equation of state nor on the mass ratio, though they do depend on stars spins. The spin dependence is approximately linear for small spins aligned with the orbital angular momentum. The quasiuniversality is a property of the conservative dynamics; nontrivial relations emerge as the binary interaction becomes tidally dominated. This analytical prediction is qualitatively consistent with new, multi-orbit numerical relativity results for the relevant case of equal-mass irrotational binaries. Universal relations are thus expected to characterize neutron star mergers dynamics. In the context of gravitational wave astronomy, these universal relations may be used to constrain the neutron star equation of state using waveforms that model the merger accurately.

Gravitational waves and mass ejecta from binary neutron star mergers: Effect of the mass ratio

“…(13), for fixed EOS (k 2 ) and fixed masses, larger q correspond to larger κ T 2 . Using the relations of [67], this implies indeed that remnants from binaries with larger q have larger angular momentum at formation.…”

“…We also find an imprint of the EOS, where in general NSs employing a soft EOS emit more energy than for a stiff EOS. Both observations can be explained in terms of the quasi-universal relations: systems with larger κ T 2 produce a more bound merger remant and release more energy [67].…”

“…Furthermore, higher mass ratios lead to smaller merger frequencies. This behavior is understood in terms of the leading order tidal coupling constant κ T 2 , and can be encoded with high precision in the quasi-universal relations proposed in [67].…”

“…Binaries with stiffer EOS are in general less bound at merger than soft EOSs binaries. This fact has been quantified in [67], that computed universal relations of the binary's binding energy and angular momentum at formation of the merger remnant as functions of the parameter κ T 2 , Eq. (13).…”

“…(13), C A is the compactness of star A defined as the ratio of the gravitational mass in isolation M A with the star's proper radius. It has been shown in [67,68] that κ T 2 is the relevant parameter encoding all the EOS information to characterize the BNS dynamics during both the merger and the postmerger phases. In addition, the GW of BNS is almost entirely determined by the mass ratio q and the κ T 2 , because the binary total mass scales trivially in absence of tides and its de- pendency in the tidal waveform is hidden in the κ T 2 .…”