Numerical-relativity simulations of long-lived remnants of binary neutron star mergers

Abstract

We analyze the properties of the gravitational wave signal emitted after the merger of a binary neutron star system when the remnant survives for more than a 80 ms (and up to 140 ms). We employ four different piecewise polytropic equations of state supplemented by an ideal fluid thermal component. We find that the post-merger phase can be subdivided into three phases: an early post-merger phase (where the quadrupole mode and a few subdominant features are active), the intermediate post-merger phase (where only the quadrupole mode is active) and the late post-merger phase (where convective instabilities trigger inertial modes). The inertial modes have frequencies somewhat smaller than the quadrupole modes. In one model, we find an interesting association of a corotation of the quadrupole mode in parts of the star with a revival of its amplitude. The gravitational wave emission of inertial modes in the late post-merger phase is concentrated in a narrow frequency region and is potentially detectable by the planned third-generation detectors. This allows for the possibility of probing not only the cold part of the equation of state, but also its dependence on finite temperature. In view of these results, it will be important to investigate the impact of various type of viscosities on the potential excitation of inertial modes in binary neutron star merger remnants.PACS numbers: 04.25.D-, 04.40. Dg, 95.30.Lz, 97.60.Jd

Instabilities in neutron-star postmerger remnants

“…Corotation of modes with rotational profiles of postmerger remnants has also been found in long-term merger simulations [16]. In particular, De Pietri et al find a convective instability in the postmerger remnant.…”

“…The initial convectively unstable region is located near the peak of the angular velocity profile, which is similar to what we find for the inner local instability using the vorticity 2-form. The highest resolution achieved in [16,69] is dx = 185 m for long-term simulations. In contrast, the simulations conducted in our study focus on the late postmerger phase.…”

“…Most previous work on differentially rotating stars assumes that the differential rotation profile corresponds to constant specific angular momentum, leading to a profile for the angular velocity, Ω, that falls off away from the rotation axis. In contrast, merger simulations suggest that the profile should be fairly flat close to the rotation axis, rising towards a maximum at some point in the remnant beyond which it tapers off towards the profile expected for a Keplerian disk [13][14][15][16]. It seems relevant to ask how this rotation profile impacts on the dynamics, e.g.…”

Gravitational-wave astrophysics from neutron star inspiral and coalescence

“…[211][212][213][214][215] and the excitation of inertial modes (under the assumption of low viscosity in the remnant) was studied in Refs. 216,217 The detection of phase transitions through GW observations of the post-merger phase was considered in. [218][219][220][221][222][223][224] For the impact of magnetic fields on the post-merger phase, see the review 225 and references therein and for the effect of a possible strong turbulent viscosity, see the review 5 and references therein, as well as the recent study using a calibrated subgrid-scale turbulence model.…”

Differentially Rotating Relativistic Stars beyond the J-Constant Law

“…More realistic rotation laws have been proposed [6], that better describe a remnant's rotational profile as reported from simulations (e.g. see [7] for a recent study). In our recent work [8], we investigate one of the new rotation laws, compare physical properties of constructed models with the "classic" KEH law to the new law and verify its suitability to describe BNS merger remnants.…”