Gravitational waves and mass ejecta from binary neutron star mergers: Effect of large eccentricities

Abstract

As current gravitational wave (GW) detectors increase in sensitivity, and particularly as new instruments are being planned, there is the possibility that ground-based GW detectors will observe GWs from highly eccentric neutron star binaries. We present the first detailed study of highly eccentric BNS systems with full (3+1)D numerical relativity simulations using consistent initial conditions, i.e., setups which are in agreement with the Einstein equations and with the equations of general relativistic hydrodynamics in equilibrium. Overall, our simulations cover two different equations of state (EOSs), two different spin configurations, and three to four different initial eccentricities for each pairing of EOS and spin. We extract from the simulated waveforms the frequency of the f-mode oscillations induced during close encounters before the merger of the two stars. The extracted frequency is in good agreement with f-mode oscillations of individual stars for the irrotational cases, which allows an independent measure of the supranuclear equation of state not accessible for binaries on quasicircular orbits. The energy stored in these f -mode oscillations can be as large as 10 −3 M ∼ 10 51 erg, even with a soft EOS. In order to estimate the stored energy, we also examine the effects of mode mixing due to the stars' offset from the origin on the f -mode contribution to the GW signal. While in general (eccentric) neutron star mergers produce bright electromagnetic counterparts, we find that for the considered cases with fixed initial separation the luminosity decreases when the eccentricity becomes too large, due to a decrease of the ejecta mass. Finally, the use of consistent initial configurations also allows us to produce high-quality waveforms for different eccentricities which can be used as a test bed for waveform model development of highly eccentric binary neutron star systems.

Interpreting binary neutron star mergers: describing the binary neutron star dynamics, modelling gravitational waveforms, and analyzing detections

“…e.g. [124,127,242,299,384,429,533,558]. In addition, it also allows to understand possible resonance effects when the orbital frequency approaches the f -mode frequency and resonant effects influencing the evolution of the system [28,483,505].…”

Damping of density oscillations in neutrino-transparent nuclear matter

“…There is evidence from simulations [49][50][51][52][53][54][55] (see also the review [56]) that eccentric binary neutron star mergers excite oscillations at frequencies above 1 kHz. We plot the dissipation times for 3 kHz and 5 kHz oscillations in Figs 8 and 9.…”

Fast Rotating Neutron Stars: Oscillations and Instabilities

“…The different oscillation patterns of a neutron star are characterized by their restoring force, e.g., p(pressure)-modes, g(gravity)-modes, i(Coriolis)-modes, s(shear)-modes or w(wave)modes. The f-mode is the fundamental mode of the p-mode sequence and it is the oscillation mode most likely to be excited in violent processes such as neutron star formation by supernova core collapse (Torres-Forné et al (2018;), the pre-merger interaction of neutron stars (see, e.g., Lai (1994); Kokkotas and Schafer (1995); Fuller and Lai (2011); Gold et al (2012); Chirenti et al (2017); Chaurasia et al (2018); Suvorov and Kokkotas (2020); Kuan et al (2021a,b)), the early-post-merger oscillations of the final object (Shibata and Taniguchi (2006); Bauswein and Janka (2012); Hotokezaka et al (2013); Bernuzzi et al (2014); Bauswein et al (2014); Lehner et al (2016); Bauswein et al (2017); Rezzolla and Takami (2016); Takami et al (2015); De Pietri et al (2018); Breschi et al (2019)). In the case that the merging neutron stars are of relatively small mass and the postmerger object is a fast spinning neutron star, the unstable f-mode oscillations can lead to its spin-down ).…”