Bulletin of the American Physical Society
2007 APS April Meeting
Volume 52, Number 3
Saturday–Tuesday, April 14–17, 2007; Jacksonville, Florida
Session M8: Focus Session: Hydrodynamics & Magnetohydrodynamics Coupled to General Relativity |
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Sponsoring Units: GGR Chair: Steve Liebling, Long Island University Room: Hyatt Regency Jacksonville Riverfront City Terrace 4 |
Sunday, April 15, 2007 3:15PM - 3:51PM |
M8.00001: General Relativistic Magnetohydrodynamic Simulations of Black Hole Accretion Disks and Jets Invited Speaker: Observations are providing increasingly detailed quantitative information about the accretion flows that power such high energy systems as X-ray binaries and Active Galactic Nuclei. These observations have been modeled in some detail by a variety of accretion scenarios, but such models rely on unavoidable assumptions such as regular flow geometry and a simple, parameterized stress. Global numerical simulations offer a way to investigate the basic physical dynamics of accretion flows without these assumptions. We have developed a fully three-dimensional general relativistic magnetohydrodynamic simulation code that evolves time-dependent inflows into Kerr black holes. The results from recent global simulations of black hole accretion disks will be reviewed, with an emphasis on the influence of the rotating hole on the disk and on jet production. [Preview Abstract] |
Sunday, April 15, 2007 3:51PM - 4:03PM |
M8.00002: Toward the evolution of magnetized differentially rotating neutron stars with AMR Matthew Anderson, Eric Hirschmann, Steven Liebling, David Neilsen The interaction of rotating compact objects endowed with magnetic fields likely plays a key role in understanding and modelling the central engines of active galactic nuclei, gamma ray bursts, and jet formation. Such simulations not only require dynamic gravitational backgrounds and relativistic fluid treatments, but also adequately resolving many time and length scales in order to capture the dynamics. Using a distributed adaptive mesh refinement infrastructure, we explore simulations of differentially rotating neutron stars both with and without magnetic fields in full 3-D. [Preview Abstract] |
Sunday, April 15, 2007 4:03PM - 4:15PM |
M8.00003: Towards Simulations of Accreting Black Holes including Magnetic Fields Miguel Megevand, Matthew Anderson, Luis Lehner, Steven Liebling, David Neilsen The study of accretion disks around compact objects is an active field in astrophysics, especially in association with relativistic jet formation. Black holes surrounded by highly magnetized accreting disks are believed to be the main source of relativistic jets. Having in mind the ultimate study of jet formation, we study relativistic magnetohydrodynamic (MHD) simulations of accretion disks around a non-spinning black hole. We use adaptive mesh refinement (AMR) within the HAD parallel infrastructure. The simulations use Cartesian coordinates in three dimensions. For solving the fluid equations we use a CENO numerical scheme with a finite difference discretization. The use of finite differences (as opposed to finite volumes) simplifies the interface with the space-time evolution in the case of dynamic backgrounds. We present tests with toroidal stationary accretion disks without magnetic field and then discuss cases with magnetic field. [Preview Abstract] |
Sunday, April 15, 2007 4:15PM - 4:27PM |
M8.00004: GRMHD simulations of differentially rotating neutron stars in dynamical spacetimes: A possible connection with GRBs Branson Stephens, Matthew Duez, Yuk Tung Liu, Stuart Shapiro, Masaru Shibata We consider the effects of magnetic fields on the evolution of differentially rotating neutron stars, which can be formed in stellar core collapse or binary neutron star coalescence. Simulations are carried out in axisymmetry using a code which integrates the coupled Einstein-Maxwell-MHD equations. Magnetic braking and the magnetorotational instability (MRI) both play important roles in the evolution. Our simulations show that the fate of the star depends on its mass and spin. The most interesting case is that of the hypermassive neutron star, for which the magnetic field leads to catastrophic collapse. The newly-formed black hole is surrounded by a hot, magnetized torus undergoing quasistationary accretion. We demonstrate that this system is a promising candidate for producing a short gamma-ray burst. [Preview Abstract] |
Sunday, April 15, 2007 4:27PM - 4:39PM |
M8.00005: General relativistic simulations of magnetic field-induced explosions from stellar core collapse Yuk Tung Liu, Masaru Shibata, Stuart Shapiro, Branson Stephens The explosion mechanism behind core-collapse supernovae remains an active area of research. In order to explore the role of magnetohydrodynamic (MHD) effects in the explosion, we perform axisymmetric simulations of magnetized, rotating cores collapsing to proto-neutron stars (PNSs) in full general relativity (dynamical spacetime). We confirm that significant differential rotation results even when the rotation of the progenitor is initially uniform. Consequently, the magnetic field is amplified both by magnetic winding and the magnetorotational instability (MRI). Following PNS formation, strong MHD outflows lead to losses of rest mass, energy, and angular momentum from the system which could play a role in rejuvenating a stalled supernova shock. [Preview Abstract] |
Sunday, April 15, 2007 4:39PM - 4:51PM |
M8.00006: General relativistic simulations of slowly rotating, magnetized stars: A perturbative metric approach Zachariah Etienne, Y.T. Liu, S. Shapiro Understanding the role general relativistic magnetohydrodynamic (GRMHD) effects play in the evolution of nascent neutron stars is a problem at the forefront of theoretical astrophysics. To this end, we performed long-term ($\sim 10^4 M$) axisymmetric simulations of differentially rotating magnetized neutron stars in the slow-rotation, weak magnetic field limit using a dynamically updated perturbative metric evolution technique. Although the perturbative metric approach yields results comparable to those obtained via a nonperturbative (BSSN) metric evolution technique, simulations performed with the perturbative metric solver require about $1/4$ the computational resources at a given resolution. This computational efficiency enabled us to observe and analyze the effects of magnetic braking and the magnetorotational instability (MRI) at very high resolution. Our GRMHD simulations demonstrate that (1) MRI is not observed unless the estimated fastest-growing mode wavelength is resolved by $\stackrel{>}{\sim} 10$ gridpoints; (2) as resolution is improved, the MRI growth {\it rate} converges, but due to the small-scale nature of MRI-induced turbulence, the maximum growth {\it amplitude} increases, but does not exhibit convergence, even at the highest resolution; and (3) independent of resolution, magnetic braking drives the star toward uniform rotation as energy is sapped from differential rotation by winding magnetic fields. [Preview Abstract] |
Sunday, April 15, 2007 4:51PM - 5:03PM |
M8.00007: A mixed pseudospectral/finite difference code for evolving spacetimes with hydrodynamic matter Matthew Duez, Lawrence Kidder, Saul Teukolsky We present a code for solving the coupled Einstein-hydrodynamics equations to evolve relativistic, self-gravitating fluids. The Einstein field equations are solved in generalized harmonic coordinates on one grid using pseudospectral methods, while the fluids are evolved on another grid using shock-capturing finite difference techniques. The grids communicate by interpolation, and filtering is used to improve the behavior near stellar surfaces. We show that the code accurately evolves equilibrium stars, both nonrotating and rapidly rotating, nonmoving and boosted. Other tests are presented, and applications to compact binaries are discussed. [Preview Abstract] |
Sunday, April 15, 2007 5:03PM - 5:15PM |
M8.00008: Force-Free Magnetohydrodynamic Waves: Non-Linear Interactions and Effects of Strong Gravity. Parker Troischt The propagation and non-linear interactions of magnetohydrodynamic waves are considered in the force-free limit, where the inertia of the conducting matter which enforces the MHD condition $E\cdot B$ = 0 can be neglected in comparison with the inertia of the electromagnetic field. By extending the analysis beyond the WKB approximation, we are able to study the non-linearities induced by a gravitational field. Here, the perturbed electromagnetic field is treated as a fluid of infinite conductivity. We calculate the scattering of a torsional (Alfven) wave by a gravitational potential, and demonstrate a nonlinear coupling with a compressive (fast) wave which is second order in the amplitude of the Alfven wave. In a cylindrically symmetric spacetime with slow rotation, the coupling is second order in $g_{t\phi } $ and first order in the amplitude of the wave. Finally, we show how spacetime curvature modifies the collision between two torsional waves, in both the weak- and strong-field regimes. [Preview Abstract] |
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