Session D16: Simulations of Black Holes, Neutron Stars, and Accretion Disks

The Impact of Stellar Compactness on Black Hole - Neutron Star Binary Mergers

Stellar disruption in black hole - neutron star mergers depends on the spin of the black hole, the mass ration of the system, and the compactness of the neutron star via its equation of state. In a previous study, we investiagted the effects of the stellar disruption on the gravitational wave emission as a function of the mass ratio for a stellar model with constant compactness. In the present study, we extend the study to investigate the effects from varying the compactness, paying attention to not only the gravitational wave emission but also to the dynamics of the apparent horizon of the black hole and its kick after the merger.   

High-accuracy waveforms for black hole-neutron star systems

The observation of gravitational waves (GWs) relies on accurate analytical waveform models. These models are also necessary for parameter estimation of the two astronomical bodies that comprise compact binaries. The production of such analytical waveform models for GW astrophysics depends on whether or not accurate numerical waveforms are available for public use. For black hole-neutron star (BHNS) merger events, there is an absence of publicly available accurate numerical waveforms. The increase in the sensitivity of GW interferometers will inevitably bring about additional detections of BHNS binary merger events. In this talk, I summarize a number of long, accurate numerical BHNS waveforms produced via the spectral Einstein code (SpEC) to better span the binary parameter space of potential BHNS merger configurations. These waveforms are publicly available as part of the Simulating eXtreme Spacetimes (SxS) collaboration’s catalog.   

Relativistic effects on neutron star fundamental-mode dynamical tides

During the final stage of the coalescence of a black hole-neutron star system, the tidal excitation of the quasi-normal modes of the neutron star can leave significant imprints on gravitational waves. Understanding this process and building a robust theoretical model is important for extracting neutron star properties from gravitational-wave data. In this talk, we will discuss how the quasi-normal modes of the neutron star in the binary environment are modified by relativistic effects, including the gravitational redshift and the frame-dragging due to the spin on the black hole. We investigate the impact of these two relativistic effects on gravitational waves by incorporating them in the SEOBNRv4T waveform model. We compare this new model with a few new black hole-neutron star simulations that are performed using a numerical relativity code Spectral Einstein Code.   

Tabulated equation of state support within IllinoisGRMHD

Presently, the best tools for understanding BNS mergers in their most extreme stages are detailed general relativistic magneto-hydrodynamic (GRMHD) simulations. Although recent  years have seen the development and improvement of accurate GRMHD codes, only a few of these are open-source. Among these is the widely used IllinoisGRMHD code, which in its present open-source state lacks the ability to simulate fluids with the use of multi-dimensional, finite temperature equation of state tables. We present new developments to IllinoisGRMHD which allow for its use with realistic, finite temperature equations of state. Our adaptation of the code includes advection of the electron fraction, which is crucial in understanding the nucleosynthetic processes associated with BNS mergers, and new conservative-to-primitive recovery routines which are based on modern solution algorithms. We discuss stringent tests of our version of IllinoisGRMHD and present detailed comparisons to other open-source GRMHD codes. We consider the evolution and amplification of magnetic fields in BNS systems described using finite temperature equations of state to showcase the full capability of our code. We find that our version of IllinoisGRMHD behaves as expected, and similar to other open-source GRMHD codes, when considering many systems of astrophysical interest.   

Smooth spectral equations of state in binary neutron star merger simulations with SpEC

Gravitational waves emitted from binary neutron star merger events can carry information about the total mass, orbital frequency, mass ratio, and compactness of the system and the stars. To reliably extract these parameters from gravitational wave observations, we need analytical waveform models calibrated to the results of high-accuracy simulations of merging binaries. However, in the Spectral Einstein Code (SpEC), high-order convergence of the simulated gravitational waveform requires smooth evolution variables, whereas most commonly used equations of state are either overly simplified or have discontinuities in the pressure function, resulting in increased errors and lower-order convergence. In this talk, we will present our results, errors, and analysis of a smooth, spectral equation of state using a series of simulations with a constant chirp mass, and varying mass ratios and resolutions.   

Simulating a magnetized neutron star with discontinuous Galerkin methods

We present a brief overview of our finite-difference-discontinuous Galerkin

hybrid method that is capable of robustly simulating a magnetized neutron star

in the Cowling approximation. We show that the scheme is able to perform

simulations over long time scales and resolve the radial oscillation frequencies

of the neutron star. The ability to perform long-term stable neutron star

evolutions is a crucial benchmark and first step towards binary merger

simulations. Our experience is that being able to maintain and resolve the

surface of a star is particularly challenging for discontinuous Galerkin

methods, while moving discontinuities are handled much more easily. The code is

publicly available at github.com/sxs-collaboration/spectre/.   

Comparison of shock capturing schemes for the discontinuous Galerkin method in GRMHD

Using our open-source code SpECTRE, we present a detailed comparison of various limiting and shock capturing strategies for the discontinuous Galerkin methods proposed in the literature applied to a set of test problems in general relativistic magnetohydrodynamics (GRMHD). We compare the standard minmod/ΛΠN limiter, the hierarchical limiter of Krivodonova, the simple WENO limiter, the HWENO limiter, and a discontinuous Galerkin-finite difference hybrid method. The goal is to evaluate the robustness and accuracy of the different strategies in order to understand which are most likely able to simulate neutron stars.   

Evolution of a magnetized accretion disk using SpECTRE

A primary goal of the open-source SpECTRE project is to simulate the disruption of neutron stars in compact-object binary systems. As a step towards this goal, we present the evolution of a relativistic magnetized accretion disk in a fixed gravitational field. The evolution uses a hybrid scheme of a discontinuous Galerkin method with embedded finite-difference regions for shock capturing.   

Can Thorne-Zytkow objects source GW1908-14 type events?

We consider scenarios where a mass gap black hole is formed as the end state of a Thorne-Zytkow object: a red giant with a neutron star in its core. We estimate the rate for such mass gap objects to subsequently merge with a black hole.