Session Y12: Numerical Relativity: Binaries with Matter

Towards realistic simulations of non-vacuum compact binaries

Binary mergers in non-vacuum spacetimes often display complex dynamics that are sensitive to the physical phenomena included in the model, and which may affect the gravitational wave signature from the system. For example, magnetic fields, cooling mechanisms, and equations of state influence the merger and post-merger evolution of compact binaries. Thus, these effects should be included in computational models that connect with astrophysical observations. In this talk we present results of neutron star evolutions with a finite-temperature equation of state in the context of binary mergers, and we also consider effects of other relevant physical phenomena.   

Electromagnetic field dynamics in Binary Neutron Stars

Neutron star mergers represent one of the most promising sources of gravitational waves (GW) within the bandwidth of advLIGO. In addition to GW, strong magnetic fields may offer the possibility of a characteristic electromagnetic signature allowing for concurrent detection. In this talk we present results from numerical evolutions of such mergers, studying the dynamics of both the gravitational and electromagnetic degrees of freedom.   

General Relativistic Simulations of Magnetized Binary Neutron Stars

Binary neutron stars are among the most important sources of gravitational waves which are expected to be detected by the current or next generation of gravitational wave detectors, such as LIGO and Virgo, and they are also thought to be at the origin of very important astrophysical phenomena, such as short gamma-ray bursts. I will report on some recent results obtained using the fully general relativistic magnetohydrodynamic code Whisky in simulating equal-mass binary neutron star systems during the last phases of inspiral, merger and collapse to black hole surrounded by a torus. I will in particular describe how magnetic fields can affect the gravitational wave signal emitted by these sources and their possible role in powering short gamma-ray bursts.   

Progress on realistic modeling of black hole-neutron star binary mergers

Black hole-neutron star (BHNS) binary mergers are important gravitational wave sources and (possibly) gamma ray burst progenitors. The current state of the art of BHNS simulations, while an impressive acheivement, is inadequate in a number of ways--most importantly in its treatment of neutron star matter and neutrino emission. We present a status report on the efforts of the Caltech-Cornell-CITA-WSU collaboration to accurately model BHNS binaries with realistic microphysics.   

Material effects in binary neutron star inspiral waveforms

Tidal effects due to the presence of matter in binary neutron star inspiral cause the gravitational wave phase to accumulate more rapidly than in binary black hole inspiral. We report a comparison of numerical waveforms from an extended set of simulations of inspiraling neutron-star binaries, computed by systematically varying the parameters of the equation of state above nuclear density. We calculate the signal strength of the difference between waveforms and obtain improved estimates of the accuracy with which second and third generation gravitational wave detectors can constrain the neutron-star equations of state. We also show how a observation of N events at varying distances improves the measurability.   

Detectability of equation of state parameters from black hole-neutron star inspiral

Gravitational waves from compact binaries containing neutron stars may provide an important source of information regarding the neutron star equation of state. In contrast to binary neutron star inspirals where finite size effects are observed from tidal interactions and post-merger oscillations, the main finite size effect in black hole-neutron star systems is tidal disruption of the neutron star and its effect on the black hole ringdown. Recently Shibata et al. performed a set of black hole-neutron star simulations where two equation of state parameters were systematically varied. Using these simulations, we discuss the accuracy to which equation of state parameters can be measured with Advanced LIGO and the proposed Einstein Telescope.   

Simulations of Astrophysical Black-Hole Formation using the Spectral Einstein Code (SpEC)

Since the first successful fully general-relativistic simulations of coalescing neutron-star binaries, researchers have steadily improved the quality of their neutron-star binary evolutions with the goal of drawing connections between neutron-star physics (such as the NS equation of state, magnetic fields, etc.) and astrophysical observables (in the form of gravitational waves and the electromagnetic signature of short gamma-ray bursts). In order to accomplish this goal, it is crucial to be able to robustly simulate the formation of black holes from collapsing nuclear matter described by a state of the art equations of state. We present the first simulations of black-hole formation from collapsing neutron-star configurations in the generalized harmonic formulation with pseudospectral methods. We employ a dual-grid approach, and solve the relativistic fluid equations using high-resolution shock-capturing techniques. In our presentation, we focus on results of long-term simulations of collapse and post-merger evolution of varied neutron star configurations.   

Binary neutron star initial data with spin

Astrophysical neutron stars are expected to be spinning. Due to the existence of millisecond pulsars we know that these spins can be substantial. As in the case of binary black holes, spins could have a large impact on the merger dynamics. Thus it is important to develop a method to set up binary neutron star initial data, where both stars can have arbitrary spins. We discuss what approximations one can make to construct binary neutron star initial data with spin. We also compare with the well known cases of corotating and irrotational neutron star binaries.   

Head-on collisions of binary white dwarf--neutron stars: Simulations in full general relativity

We simulate head-on collisions from rest at large separation of binary white dwarf--neutron stars (WDNSs) in full general relativity (GR). We focus on WDNSs whose total mass exceeds the maximum mass of a NS, and our goal is to determine their fate. A full GR hydrodynamic computation of realistic WDNS systems is prohibitive due to the large range of dynamical time-scales and length-scales involved. For this reason, we construct an equation of state (EOS) that mimics realistic NS EOSs while, at the same time, reduces the size of WDs. We call these scaled-down WD models ``pseudo-WDs (pWDs)". Using pWDs, we can study WDNSs via a sequence of simulations where the size of the pWD gradually increases toward realistic cases. We perform simulations that study the effects of both the NS mass and the pWD compaction separately. We find that all remnant masses exceed the maximum mass of our cold EOS ($1.92 M_\odot$), but no case leads to prompt collapse to a black hole. This outcome arises because the final configurations are hot. All cases settle into spherical, quasiequilibrium configurations consisting of a cold NS core surrounded by a hot mantle, resembling Thorne-Zytkow objects. Our study indicates that the likely outcome of a head-on collision of a realistic, massive WDNS system will be a Thorne-Zytkow-like object.