Session R18: Numerical Relativity: Neutron Stars and Other Sources with Matter

Live

With the recent observations of gravitational wave signals from binary neutron star mergers, relativistic hydrodynamics has become testable by numerical simulations. Numerous simulations currently exist exploring parameters of binaries such as mass ratio, distance, spins, and equations of states of the constituent Neutron stars. Different numerical approaches exist such as the BSSN or BSSNOK formalisms, and the conformally flat approximation that solve the Einstein equation efficiently on computers. In this work, the open source code Einstein Toolkit is used to study binary neutron star merger simulations for various equations of state and relativistic solutions in an attempt to regenerate the observed signal. We also add neutrino transport to more accurately describe the dynamical merger process. The goal is to better constrain the equation of state of neutron matter.   

Live

We have made a new infrastructure, named Elliptica, to construct initial data for various compact objects. Elliptica is an open-source code uses multi-domain spectral method with Schur complement domain decomposition and direct solver to solve elliptic equations with arbitrary boundary conditions. Here, we present our first results about the construction of initial data for black hole-neutron star binaries with arbitrary spin.   

Live

The concurrent detection of the collision of neutron stars in gravitational waves and the electromagnetic spectrum provided a unique opportunity to constrain the equation of state of neutron star matter. With more collisions expected to be detected during LIGO's 3rd observation run, these constraints are expected to be tightened further. While the observed neutron star collision, and the majority of expected signals, were of neutron stars in circular orbit about each other, very dense environment can lead to eccentric neutron star mergers. During these mergers the stars can approach much closer than during a circular inspiral changing the gravitational wave signal and allowing new effects to be probed. I report on activity of the NCSA gravity group to simulate binary neutron star mergers on eccentric orbits using the Einstein Toolkit code.   

Live

There are various theoretical motivations for expecting a phase transition in matter at extreme densities above supranuclear accompanied by hopes that gravitational wave observations may reveal the properties of such a transition. We consider a generic form of first order phase transition using a piecewise polytrope equation of state, and evolve both isolated neutron stars and neutron star binaries looking at dynamical effects. Of particular interest are effects that may be observable.   

Sub-Grid Viscosity in Neutron Star Simulations with SpEC

Viscosity in neutron star simulations is capable of replicating the angular momentum transportation and heating of some instabilities that are currently too costly to capture in simulations, yet affect the gravitational wave, matter outflows, neutrino emission, and remnant of neutron star binaries. In this talk, we will present the results of SpEC simulations using sub-grid viscosity models, and compare the costs and behaviors of different viscosity models.   

Magnetic Braking and Damping of Differential Rotation in Massive Stars

Fragmentation of highly differentially rotating massive stars that undergo collapse has been suggested as a possible channel for binary black hole formation. Such a scenario could explain the formation of the new population of massive black holes detected by the LIGO/VIRGO gravitational wave laser interferometers. We probe that scenario by performing general relativistic magnetohydrodynamic simulations of differentially rotating massive stars supported by thermal radiation pressure plus a gas pressure perturbation. The stars are initially threaded by a dynamically weak, poloidal magnetic field confined to the stellar interior. We find that magnetic braking and turbulent viscous damping via magnetic winding and the magnetorotational instability in the bulk of the star redistribute angular momentum, damp differential rotation and induce the formation of a massive and nearly uniformly rotating inner core surrounded by a Keplerian envelope. The core + disk configuration evolves on a secular timescale and remains in quasi-stationary equilibrium until the termination of our simulations. Our results suggest that the high degree of differential rotation required for $m=2$ seed density perturbations to trigger gas fragmentation and binary black hole formation.   

Not Participating

The GW170817 event was the first detection of a binary neutron star (BNS) merger in both gravitational waves and light, thus receiving worldwide attention. Recently, a second gravitational wave detection from a heavier BNS was announced: GW190425. In this work we model the equation of state (EOS) of neutron stars and to understand how the magnetic field is amplified during the BNS evolution. We construct initial data with the LORENE code and choose three EOS, constrained by the GW170817 and GW190425 events. We simulate each event with the Einstein Toolkit, considering first the same EOS for the binary, then allowing different EOS for individual stars. We add dipolar magnetic fields, modeling the magnetospheres with the general relativistic force-free electrodynamics code GiRaFFE. We monitor the mass density, the gravitational waves, and for the simulations with magnetic field, the electromagnetic Poynting luminosity. We quantify the relationship between the chosen equations of state, the gravitational wave signal, and the collimation of the electromagnetic field. If the merger produces a black hole, we look for the incipience of jets. Our work will help with understanding matter at high density, and highly energetic sources of both gravitational and electromagnetic radiation.