Bulletin of the American Physical Society
2006 APS April Meeting
Saturday–Tuesday, April 22–25, 2006; Dallas, TX
Session H11: Relativistic Simulations of Neutron Stars, Black Holes, and Plasmas |
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Sponsoring Units: GGR Chair: Harald Pfeiffer, California Institute of Technology Room: Hyatt Regency Dallas Cumberland E |
Sunday, April 23, 2006 8:30AM - 8:42AM |
H11.00001: Binary neutron stars: Equilibrium models beyond spatial conformal flatness Koji Uryu, Francois Limousin, John Friedman, Eric Gourgoulhon, Masaru Shibata Equilibria of binary neutron stars in close circular orbits are computed numerically in a waveless approximation to general relativity. The new formulation exactly solves the Einstein-Euler system written in 3+1 form on a spacelike hypersurface. All components of the field equation are written elliptic equations, and hence all metric components, including the spatial metric, have Coulomb-type fall off. We choose the time-derivative of conformal three-metric to vanish on a spacelike hypersurface for a waveless condition, and impose helical symmetry for the other quantities. Two independent numerical codes, one based on a finite difference method, the other on a spectral method, are developed, and solution sequences that model inspiraling binary neutron stars during the final several orbits are successfully computed. The binding energy of the system near its final orbit deviates from earlier results of third post-Newtonian and of spatially conformally flat calculations. The new solutions may serve as initial data for merger simulations and as members of quasiequilibrium sequences to generate gravitational wave templates, and may improve estimates of the gravitational-wave cutoff frequency set by the last inspiral orbit. [Preview Abstract] |
Sunday, April 23, 2006 8:42AM - 8:54AM |
H11.00002: Models of helically symmetric binary systems Jocelyn Read, Shin'ichirou Yoshida, Benjamin Bromley, Koji Uryu, John Friedman We report results from helically symmetric scalar field models and first results from a convergent helically symmetric binary neutron star code; these are models stationary in the rotating frame of a source with constant angular velocity $\Omega$. In the scalar field models and the neutron star code, helical symmetry leads to a system of mixed elliptic-hyperbolic character. The scalar field models involve nonlinear terms of the form $\psi^ 3$, $(\nabla\psi)^2$, and $\psi\Box\psi$ that mimic nonlinear terms of the Einstein equation. Convergence is strikingly different for different signs of each nonlinear term; it is typically insensitive to the iterative method used; and it improves with an outer boundary in the near zone. In the neutron star code, convergence has been achieved only for an outer boundary less than $\sim 1$ wavelength from the source or for a code that imposes helical symmetry only inside a near zone of that size. The inaccuracy of helically symmetric solutions with appropriate boundary conditions should be comparable to the inaccuracy of a waveless approximation that neglects gravitational waves; and the (near zone) solutions we obtain for waveless and helically symmetric BNScodes with the same boundary conditions nearly coincide. [Preview Abstract] |
Sunday, April 23, 2006 8:54AM - 9:06AM |
H11.00003: Neutron Star - Black Hole Mergers: Gravitational waves and implications for short GRBs Pablo Laguna, Fred Rasio, Emmanouela Rantsiou, Shiho Kobayashi A promising source of gravitational radiation in LIGO's sensitivity window is the inspiral and coalescence of a binary system consisting of a BH and a NS. There is additional interest in BH-NS binaries because they could play a key role as the central engine for short GRBs. If this is indeed the scenario, BH-NS binaries will give us a unique opportunity for \emph{multi-messenger} astronomy, namely mergers that would be able to be seen both in the electromagnetic and gravitational windows. We present results of general relativistic SPH simulations of BH-NS merger, including estimates of the gravitational waves emitted during these astronomical events. [Preview Abstract] |
Sunday, April 23, 2006 9:06AM - 9:18AM |
H11.00004: Simulations of magnetized, differentially rotating neutron stars in full general relativity Branson C. Stephens, Matthew D. Duez, Yuk Tung Liu, Stuart L. Shapiro, Masaru Shibata Many problems at the forefront of theoretical astrophysics require the treatment of magnetized fluids in dynamical, strongly curved spacetimes. In order to tackle such problems, we have recently developed a code which evolves the full Einstein-Maxwell-MHD system of equations. We employ this code to track the evolution of magnetized hypermassive neutron stars (HMNSs), which are equilibrium configurations supported against collapse by rapid differential rotation. HMNSs may form as possible transient remnants of binary neutron star mergers or through core collapse. We find that secular angular momentum transport due to magnetic braking and the magnetorotational instability results in the collapse of the HMNSs to rotating black holes surrounded by massive accretion tori. For comparison, we also evolve non-hypermassive but differentially rotating neutron stars with magnetic fields. Though we do not find collapse, an extended, torus-like structure does form for stars with excessive spin. [Preview Abstract] |
Sunday, April 23, 2006 9:18AM - 9:30AM |
H11.00005: Relativistic magnetohydrodynamics with black hole excision: Application to short gamma-ray bursts Yuk Tung Liu, Matthew Duez, Stuart Shapiro, Masaru Shibata, Branson Stephens A hypermassive neutron star (HMNS) is a possible transient formed after the merger of a neutron star binary. In our latest magnetohydrodynamic simulations in full general relativity, we find that a magnetized HMNS undergoes `delayed' collapse to a rotating black hole (BH) as a result of angular momentum transport via magnetic braking and the magnetorotational instability. The outcome is a BH surrounded by a massive, hot torus with a collimated magnetic field. To follow the subsequent evolution of this system, we continue the evolution using a black hole excision technique. We find that the system quickly settles down to a quasi-stationary state. The torus is hot and accretes onto the BH quasi-steadily. This BH-torus system is a promising central engine for the short-duration gamma-ray bursts. This model predicts that a short gamma-ray burst formed in this scenario should accompany a burst of gravitational waves and neutrinos. [Preview Abstract] |
Sunday, April 23, 2006 9:30AM - 9:42AM |
H11.00006: MHD in general relativity David Neilsen, Eric Hirschmann, Matthew Anderson, Luis Lehner, Steven Liebling Magnetic fields play an important role in many astrophysical phenomena. In this talk we present our method for solving the ideal MHD equations in general relativity. We discuss black hole excision, AMR for both fluid and geometric variables, and evolutions with dynamic geometry. Preliminary results on a black hole spacetime will be presented. [Preview Abstract] |
Sunday, April 23, 2006 9:42AM - 9:54AM |
H11.00007: Relativistic MHD and black hole excision: Formulation and initial tests Eric Hirschmann, David Neilsen, Steven Millward It is widely expected that strong magnetic fields are an important factor in the dynamics of many relativistic and energetic astrophysical phenomena. As a consequence, several groups have begun work on developing approaches to simulating the equations of relativistic magnetohydrodynamics. We present one such approach, including our formalism, high resolution shock capturing method on multiple coordinate patches, primitive variable solver and solenoidal constraint solver. We present tests and results in flat space. [Preview Abstract] |
Sunday, April 23, 2006 9:54AM - 10:06AM |
H11.00008: Relativistic magnetohydrodynamics with adaptive mesh refinement Matthew Anderson, Eric Hirschmann, Steve Liebling, David Neilsen Simulations involving relativistic fluids frequently require the numerical treatment of a large range of time and length scales. Parallel adaptive mesh refinement (AMR) techniques can greatly reduce the computational expense of such simulations by adaptively placing greater mesh resolution where needed in order to control error size. We present several relativistic magnetohydrodynamic simulations in 3+1 dimensions using parallel, vertex centered AMR. We focus on several special relativistic tests: the relativistic rotor, spherical shock, and Komissarov shocktube. Performance measures and speedups are also presented. [Preview Abstract] |
Sunday, April 23, 2006 10:06AM - 10:18AM |
H11.00009: Introducing Flow-er: a Hydrodynamics Code for Relativistic and Newtonian Flows Patrick Motl, Ignacio Olabarrieta, Joel Tohline We present a new numerical code (Flow-er) for calculating astrophysical flows in 1, 2 or 3 dimensions. We have implemented equations appropriate for the treatment of Newtonian gravity as well as the general relativistic formalism to treat flows with either a static or dynamic metric. The heart of the code is the recent non-oscillatory central difference scheme by Kurganov and Tadmor (2000). With this technique, we do not require a characteristic decomposition or the solution of Riemann problems that are required by most other high resolution, shock capturing techniques. Furthermore, the KT scheme naturally incorporates the Method of Lines, allowing considerable flexibility in the choice of time integrators. We have implemented several interpolation kernels that allow us to choose the spatial accuracy of an evolution. Flow-er has been tested against an independent implementation of the KT scheme to solve the relativistic equations in 1d - which we also describe. Flow-er can serve as the driver for the hydrodynamical portion of a simulation utilizing adaptive mesh refinement or on a unigrid. In addition to describing Flow-er, we present results from several test problems. [Preview Abstract] |
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