Session H8: Numerical Relativity: Neutron Stars and Black Holes

Binary neutron stars 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. Spin periods of a few dozen milliseconds will influence the inspiral and potentially also the merger of binary neutron stars. We have developed a new method to set up binary neutron star initial data, where both stars can have arbitrary spins. We use these new initial data as a starting point for numerical simulations. We present preliminary results where we compare simulations of equal mass binaries with and without spin.   

Simulations of Binaries Neutron Star with Arbitrary Spins using the Einstein Toolkit

Binary neutron stars are among the most important sources of the gravitational waves and potential engines of short gamma-ray bursts. Observations of millisecond pulsars suggest that neutron stars can have substantial spins pointing in arbitrary directions. Thus realistic numerical simulations of the late inspiral and merger require initial data with arbitrary spins. We are presenting method of a constructing such an initial data by combining a solutions of two single rotating neutron stars. We report on our recent progress in simulating the merger of neutron star binaries with an arbitrary spins using Einstein Toolkit.   

Binary NS simulations using SpEC

NSNS binaries are expected to be one of the major sources of gravitational radiation detectable by Advanced LIGO. Together with neutrinos, gravitational waves are our only means to learn about the processes deep within a merging pair of NS, shedding light on the as yet poorly understood, equation of state governing matter at nuclear densities and beyond. We report on binary neutron star simulations using the Spectral Einstein Code (SpEC) developed by the Caltech-Cornell-CITA-WSU collaboration. We simulate the inspiral through many orbits, follow the post-merger evolution, and compute the full gravitational wave signal. We provide estimates on the accuracy required for the LIGO scientific goals of constraining EOS parameters.   

Tidal excitation of normal modes in eccentric binary neutron stars

Neutron star binaries offer a rich phenomenology in terms of gravitational waves and merger remnants. However, most general relativistic studies have been performed for nearly circular binaries, with the exception of head-on collisions. We present the first numerical relativity investigation of mergers of eccentric neutron-star binaries that probes the regime between head-on and circular. Upon variation of the initial eccentricity, covering cases from direct plunge to more adiabatic inspiral, we study the outcome of a binary composed of two $1.4M_\odot$ neutron stars. We characterize the gravitational wave emission, the internal dynamics of the stars and the properties of the merger remnant. In addition to gravitational waves generated by the orbital motion, we find that the signal also contains a strong component due to stellar oscillations ($f$-modes) induced by tidal forces, extending a classical result for Newtonian binaries. Such signatures may be used to constrain the NS equation of state. With the exception of extreme eccentricities (near head-on collisions) the merger leads generically to rather massive disks, which in some cases can be on the order of 10\% of the total initial mass. All merger remnants form a black hole making such encounters a plausible SGRB engine.   

Neutron star evolutions using tabulated equations of state with a new execution model

The addition of nuclear and neutrino physics to general relativistic fluid codes allows for a more realistic description of hot nuclear matter in neutron star and black hole systems. This additional microphysics requires that each processor have access to large tables of data, such as equations of state, and in large simulations the memory required to store these tables locally can become excessive unless an alternative execution model is used. In this talk we present neutron star evolution results obtained using a message driven multi-threaded execution model known as ParalleX as an alternative to using a hybrid MPI-OpenMP approach. ParalleX provides the user a new way of computation based on message-driven flow control coordinated by lightweight synchronization elements which improves scalability and simplifies code development. We present the spectrum of radial pulsation frequencies for a neutron star with the Shen equation of state using the ParalleX execution model. We present performance results for an open source, distributed, nonblocking ParalleX-based tabulated equation of state component capable of handling tables that may even be too large to read into the memory of a single node.   

Fully General Relativistic Simulations of Magnetized Neutron Star--Black Hole Binary Mergers

As a neutron star (NS) is disrupted by black hole (BH) tidal fields at the end of a BH--NS binary inspiral, its magnetic fields will be stretched and amplified. These magnetic fields may impact the gravitational waveforms, merger evolution and mass of the remnant disk. Formation of highly-collimated magnetic field lines in the remnant may launch relativistic jets, driving an SGRB. We analyze this scenario through fully general relativistic, magnetohydrodynamic BH--NS simulations from inspiral through merger and disk formation. Multiple seed magnetic field configurations are chosen, starting with both nonspinning and moderately-spinning ($a/M$=0.75) BHs aligned with the orbital angular momentum. Only strong ($B_{max}\sim10^{17}$G) initial magnetic fields in the NS significantly influence merger dynamics, enhancing the remnant disk mass by 100\% and 40\% in the nonspinning and spinning BH cases, respectively. We find that detecting the effects of even strong magnetic fields may be challenging for Advanced LIGO. While there is no evidence of outflows during the preliminary simulations we have explored, longer disk evolutions, improved resolution and different field topologies will be required to more thoroughly assess the plausibility of BHNS binaries as SGRB progenitors.   

Relativistic MHD in dynamical spacetimes: Improved EM gauge condition for AMR grids

We recently developed a new GRMHD code with AMR that evolves the electromagnetic (EM) vector potential $A_i$ instead of the magnetic fields directly. Evolving $A_i$ enables one to use any interpolation scheme on refinement level boundaries and still guarantee that the magnetic field remains divergenceless. As in classical EM, a gauge choice must be made when evolving $A_i$, and we chose a straightforward ``algebraic'' gauge condition to simplify the $A_i$ evolution equation. However, magnetized black hole-neutron star (BHNS) simulations in this gauge exhibit unphysical behavior, including the spurious appearance of strong magnetic fields on refinement level boundaries. This spurious behavior is exacerbated when matter crosses refinement boundaries during tidal disruption of the NS. We demonstrate via an eigenvalue analysis and a numerical study that zero-speed modes in the algebraic gauge, coupled with the frequency filtering that occurs on refinement level boundaries, are responsible for the creation of spurious magnetic fields. We show that the EM Lorenz gauge exhibits no zero-speed modes, and as a consequence, spurious magnetic effects are quickly propagated away, allowing for long-term, stable magnetized BHNS evolutions.   

Eccentric Compact Object Mergers

Mergers of black holes and neutron stars are expected to be an important source of gravitational radiation for upcoming observatories. Such mergers are also a leading candidate for short gamma-ray burst progenitors and may be source for other electromagnetic counterparts. An interesting class of compact object binaries are those that form in dense stellar regions such as globular clusters and may merge with significant eccentricity. We present results from general-relativistic hydrodynamics simulations that are performed in order to explore the dynamics and possible observational signatures of such systems.   

Black Hole-Neutron Star Mergers for 10 Solar Mass Black Holes

Black hole-neutron star (BHNS) mergers are expected to be observed by gravitational wave detectors within the next few years, and are also thought to be promising candidates as short gamma-ray burst progenitors. The parameters of BHNS binaries which affect the dynamics of the merger the most (black hole mass and spin, nuclear equation of state) are highly uncertain. For the black hole mass, population synthesis models indicate that fairly massive black holes ($>10M_\odot$) are probably the norm in such systems. Numerical simulations of BHNS mergers in general relativity have however been focused on lower mass black holes ($\sim 3-7M_\odot$). I will present recent numerical simulations of BHNS mergers for $10 M_\odot$ black holes, and show how they differ from lower mass systems in the emitted gravitational waveform as well as in their ability to form the massive discs required to power short gamma-ray bursts.