Session L08: Numerical Relativity: Binary Black Holes and Neutron Stars

Live

We perform magnetohydrodynamic simulations of accreting, equal-mass binary black holes in full general relativity focusing on the dynamical formation and evolution of minidisks. We find that during the late inspiral the sizes of minidisks are determined by the interplay between the tidal field and the effective innermost stable orbit around each black hole. In particular, we show evidence that minidisks form when the Hill sphere around each black hole is significantly larger than the black hole's effective innermost stable orbit. As the binary inspirals, the Hill sphere radius decreases, and hence minidisks and their associated electromagnetic signatures will disappear prior to merger when there are no more stable orbits within the Hill sphere. The disappearance of a hard electromagnetic component in the spectrum of such systems may provide a smoking gun signature of merging black hole binaries.   

Live

Improved ground-based gravitational wave detectors and highly anticipated space-based detectors are expected to provide us with a wealth of gravitational wave observations. In order to observe and characterize these signals, we require highly accurate template waveforms, often generated from models trained on numerical relativity waveforms. This relies upon having a catalog of numerical relativity waveforms which densely covers the possible parameter space. Unfortunately, numerical relativity simulations are time consuming and computationally expensive, limiting the number of simulations that can be performed. In order to reduce the necessary parameter space coverage and optimize simulation placement, we explore the impact of the secondary spin in unequal mass black hole binaries. As the mass ratio of a binary increases, the spin of the secondary black hole is expected to become less significant. This talk explores the ability of current and future detectors to distinguish the spin of the secondary black hole, allowing us to avoid densely filling indistinguishable regions of parameter space and instead focus our resources on more impactful simulations.   

Live

The black hole uniqueness theorems imply that the final state of a merged binary black-hole system is a Kerr black hole. Unlike this simple geometry, the black hole immediately after the merger is highly distorted. Multipole moments on dynamical and isolated horizons offer a natural avenue for describing the decay of this distortion. We construct a spatially gauge-invariant version of horizon multipole moments in the Spectral Einstein Code (SpEC), and explore their evolution in the ringdown phase. We also compare these moments to the waveform at null infinity, relating the strong field regime to the radiation zone.   

Live

We present a new study of remnant black hole properties from 13 binary black hole systems, numerically evolved using the Spectral Einstein Code. The mass, spin, and recoil velocity of each remnant were computed locally from apparent horizon data and asymptotically from the Bondi data $(h, \psi_4, \psi_3, \psi_2, \psi_1)$ made available at future null infinity with a Cauchy characteristic evolution. We compare these two independent measurements of the remnant properties, giving insight into how well asymptotic waveforms reveal local information of the remnant black hole in numerical relativity. This study highlights the importance of fixing the BMS frame of numerically determined waveforms.   

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 the mass ratio, distance, spins, and the equations of state (EOSs) of the constituent neutron stars. Many different numerical approaches exist such as the BSSN or BSSNOK formalisms, and the conformally flat approximation that can solve the Einstein equation efficiently. In this work we will be doing full GR three-dimensional hydrodynamics simulation by first constructing initial data using Lorene followed by simulating the merger with Einstein Toolkit. The goal is to study binary neutron stars described by various EOSs and their effects on the observed GW waveform as the merger happens. We also perform tests on the EOSs using the Rotating neutrons star code to check their validity under modern standards of mass limits.   

Live

We have developed a new code, called Elliptica, to construct initial data for various compact objects. Here we construct initial data for a black hole-neutron star system where the black hole interior is excised. Our aim is to evolve these initial data with a code like BAM that uses the moving puncture method. We thus have to fill the interior of the black hole with valid data before we can start the evolution. Here, we present first results from our effort to evolve these data, such as trajectories, constraint violations, and gravitational waves.   

Live

The launching of a new era of multimessenger astrophysics has led to a particularly interesting challenge of constraining the equation of state (EOS) of the nuclear matter inside the neutron star core. We have modified the LORENE code to construct unequal mass BNS initial data for different equations of states and developed an initial data library for use in dynamical simulations. We have used our initial data to launch dynamical runs of BNS mergers using the Einstein Toolkit. Here, we discuss our analysis of the dynamics of the merger for different mass ratios and piecewise polytropic EOSs and will be focusing on how different initial properties of the neutron stars affect the amount of mass ejected during and after the merger.   

Live

We report on the RIT Binary Neutron Star Initial Data Library, a publicly available repository of quasi-equilibrium data generated using the Lorene code, for use in dynamical simulations of merging neutron stars. Our initial data include both equal-mass and unequal-mass configurations, for a variety of neutron star equation of state models including piecewise polytropes as well as tabulated models motivated by nuclear physics calculations. We will discuss the step required to construct such models, including the most robust ways to generate a sequence of configurations at different binary separations.   

Live

Accretion disks around BHs are an under-studied potential GW source. The hydrodynamic Papaloizou-Pringle Instability (PPI) can cause persistent orbiting matter clumps to grow and produce copious GWs. Via full numerical relativity simulations of self-gravitating disks, we have extended the understanding of these BH-disk systems in two new ways. First, we conducted the first-ever study of the PPI around spinning BHs ($a/M = 0.7$). We found that, in addition to slightly shifting orbital frequencies, prograde spin can reduce the accretion rate and extend GW signal lifetimes. Systems of $10 M_\odot$ - relevant for BHNS mergers - could be detectable by Cosmic Explorer out to $\sim300$ Mpc, while DECIGO (LISA) could detect systems of $1000 M_\odot$ ($10^5M_\odot$) - relevant for disks forming in collapsing supermassive stars - out to cosmological redshift of $z\sim5$ ($z\sim 1$). Second, we investigated the impact of magnetic fields on the PPI.