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

Abstract

We present new (3+1)D numerical relativity simulations of the binary neutron star (BNS) merger and postmerger phase. We focus on a previously inaccessible region of the binary parameter space spanning the binary's mass-ratio q ∼ 1.00 − 1.75 for different total masses and equations of state, and up to q ∼ 2 for a stiff BNS system. We study the mass-ratio effect on the gravitational waves (GWs) and on the possible electromagnetic emission associated to dynamical mass ejecta. We compute waveforms, spectra, and spectrograms of the GW strain including all the multipoles up to l = 4. The mass-ratio has a specific imprint on the GW multipoles in the late-inspiral-merger signal, and it affects qualitatively the spectra of the merger remnant. The multipole effect is also studied by considering the dependency of the GW spectrograms on the source's sky location. Unequal mass BNSs produce more ejecta than equal mass systems with ejecta masses and kinetic energies depending almost linearly on q. We estimate luminosity peaks and light curves of macronovae events associated to the mergers using a simple approach. For q ∼ 2 the luminosity peak is delayed for several days and can be up to four times larger than for the q = 1 cases. The macronova emission associated with the q ∼ 2 BNS is more persistent in time and could be observed for weeks instead of few days (q = 1) in the near infrared. Finally, we estimate the flux of possible radio flares produced by the interaction of relativistic outflows with the surrounding medium. Also in this case a large q can significantly enhance the emission and delay the peak luminosity. Overall, our results indicate that BNS merger with large mass ratio have EM signatures distinct from the equal mass case and more similar to black hole -neutron star binaries.

Simulating Binary Neutron Stars with Hybrid Equation of States: Gravitational Waves, Electromagnetic Signatures and Challenges for Numerical Relativity

“…Compared to the SLy simulations in Refs. [52,70] this is only about ∼ 15 to 40% of the SLy simulations. This means that if even half of the disk would be ejected (see Fig.…”

Kilohertz gravitational waves from binary neutron star remnants: Time-domain model and constraints on extreme matter

“…Two coupling frequencies are identified from secondary peaks of the spectra, see e.g. [21,22,73,74] (and figures below in this paper); we indicate them asf 2±0 following the notation of [24]. Although we will often refer to discrete frequencies (spectral peaks), we stress that the GW frequency is not constant but evolves (chirp-like) as the remnant becomes more compact and eventually collapses (see SLy data in Fig.…”

“…KiloHertz GWs contain the imprint of the merger remnant dynamics. The main signature is a short GW transient peaking at a few characteristic frequencies, the dominant one being associated with twice the rotation frequency of the remnant NS at f 2 > f mrg [16,[21][22][23][24][25][26][27][28][29][30]. The transient is more luminous for short-lived remnant than for longlived; an absolute upper limit to the energy per unit mass is 0.126( M 2.8M ) M c 2 , where M is the binary mass [12].…”

Search for Post-merger Gravitational Waves from the Remnant of the Binary Neutron Star Merger GW170817

“…For example, short-lived hypermassive NSs are expected to emit a few percent of a solar mass in GW energy (e.g., see the two lower dashed lines in Figure 1 that represent 1% and 10% of a solar mass; Kiuchi et al 2009;Clark et al 2014;Bernuzzi et al 2015b;Endrizzi et al 2016;Dietrich et al 2017aDietrich et al , 2017bFeo et al 2017), while our minimum 50% efficiency is   E M c 4.8 Corsi & Mészáros 2009). Figure 1 shows the h rss 50% for the considered waveform models as a function of the waveform's signal-weighted frequency.…”