Impact of bulk viscosity on the post-merger gravitational-wave signal from merging neutron stars
ORAL
Abstract
In the violent post-merger of binary neutron-star mergers strong oscillations are present that impact the emitted gravitational-wave (GW) signal and allow for violations of the chemical equilibrium promoted by weak-interactions, thus leading to a nonzero bulk viscosity. We present the first simulations of binary neutron-star mergers employing the self-consistent and second-order formulation of the equations of relativistic hydrodynamics for dissipative fluids proposed by Müller, Israel and Stewart. With the spirit of obtaining a first assessment of the impact of bulk viscosity on the structure and radiative efficiency of the merger remnant we adopt a simplified but realistic approach for the viscosity, which we assume to be determined by direct and modified Urca reactions. At the same time, we explore the possible behaviours by considering three different scenarios of low, medium, and high bulk viscosity. In this way, we find that large values of the bulk viscosities damp the collision-and-bounce oscillations that characterize the dynamics of the stellar cores right after the merger. At the same time, large viscosities tend to preserve the m = 2 deformations in the remnant, thus leading to a comparatively more efficient GW emission and to changes in the post-merger spectrum that can be up to 100 Hz in the case of the most extreme configurations.
*Partial funding comes from the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt as part of the strategic R&D collaboration with Goethe University Frankfurt, from the State of Hesse within the Research Cluster ELEMENTS (Project ID 500/10.006), by the ERC Advanced Grant (Grant No. 884631) and the Deutsche Forschungsgemeinschaft through the CRC-TR 211 – project number 315477589. LR acknowledges the Walter Greiner Gesellschaft zur Förderung der physikalischen Grundlagenforschung e.V. through the Carl W. Fueck Laureatus Chair. The simulations were performed on HPE Apollo HAWK at the High Performance Computing Center Stuttgart (HLRS) under the grant BNSMIC. We also acknowledge support from NSF awards AST-2009330, OAC-2031744, PHY-1806596, and PHY-2110352, NASA TCAN awards NNH17ZDA001N and 80NSSC24K0100.
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Publication:Impact of bulk viscosity on the post-merger gravitational-wave signal from merging neutron stars - https://arxiv.org/abs/2307.10464 Numerical modelling of bulk viscosity in neutron stars - https://arxiv.org/abs/2311.13027
