454 research outputs found
MAGMA: a 3D, Lagrangian magnetohydrodynamics code for merger applications
We present a new, completely Lagrangian magnetohydrodynamics code that is
based on the SPH method. The equations of self-gravitating hydrodynamics are
derived self-consistently from a Lagrangian and account for variable smoothing
length (``grad-h''-) terms in both the hydrodynamic and the gravitational
acceleration equations. The evolution of the magnetic field is formulated in
terms of so-called Euler potentials which are advected with the fluid and thus
guarantee the MHD flux-freezing condition. This formulation is equivalent to a
vector potential approach and therefore fulfills the
-constraint by construction. Extensive tests in
one, two and three dimensions are presented. The tests demonstrate the
excellent conservation properties of the code and show the clear superiority of
the Euler potentials over earlier magnetic SPH formulations.Comment: 18 pages, 17 Figures, a high resolution copy of the paper can be
found at http://www.faculty.iu-bremen.de/srosswog/MAGMA.pd
Evolving relativistic fluid spacetimes using pseudospectral methods and finite differencing
We present a new code for solving the coupled Einstein-hydrodynamics
equations to evolve relativistic, self-gravitating fluids. The Einstein field
equations are solved on one grid using pseudospectral methods, while the fluids
are evolved on another grid by finite differencing. We discuss implementation
details, such as the communication between the grids and the treatment of
stellar surfaces, and present code tests.Comment: To appear in the Proceedings of the Eleventh Marcel Grossmann Meetin
Orbiting binary black hole evolutions with a multipatch high order finite-difference approach
We present numerical simulations of orbiting black holes for around twelve
cycles, using a high-order multipatch approach. Unlike some other approaches,
the computational speed scales almost perfectly for thousands of processors.
Multipatch methods are an alternative to AMR (adaptive mesh refinement), with
benefits of simplicity and better scaling for improving the resolution in the
wave zone. The results presented here pave the way for multipatch evolutions of
black hole-neutron star and neutron star-neutron star binaries, where high
resolution grids are needed to resolve details of the matter flow
The Influence of Thermal Pressure on Equilibrium Models of Hypermassive Neutron Star Merger Remnants
The merger of two neutron stars leaves behind a rapidly spinning hypermassive
object whose survival is believed to depend on the maximum mass supported by
the nuclear equation of state, angular momentum redistribution by
(magneto-)rotational instabilities, and spindown by gravitational waves. The
high temperatures (~5-40 MeV) prevailing in the merger remnant may provide
thermal pressure support that could increase its maximum mass and, thus, its
life on a neutrino-cooling timescale. We investigate the role of thermal
pressure support in hypermassive merger remnants by computing sequences of
spherically-symmetric and axisymmetric uniformly and differentially rotating
equilibrium solutions to the general-relativistic stellar structure equations.
Using a set of finite-temperature nuclear equations of state, we find that hot
maximum-mass critically spinning configurations generally do not support larger
baryonic masses than their cold counterparts. However, subcritically spinning
configurations with mean density of less than a few times nuclear saturation
density yield a significantly thermally enhanced mass. Even without decreasing
the maximum mass, cooling and other forms of energy loss can drive the remnant
to an unstable state. We infer secular instability by identifying approximate
energy turning points in equilibrium sequences of constant baryonic mass
parametrized by maximum density. Energy loss carries the remnant along the
direction of decreasing gravitational mass and higher density until instability
triggers collapse. Since configurations with more thermal pressure support are
less compact and thus begin their evolution at a lower maximum density, they
remain stable for longer periods after merger.Comment: 20 pages, 12 figures. Accepted for publication in Ap
The Final Fate of Binary Neutron Stars: What Happens After the Merger?
The merger of two neutron stars usually produces a remnant with a mass
significantly above the single (nonrotating) neutron star maximum mass. In some
cases, the remnant will be stabilized against collapse by rapid, differential
rotation. MHD-driven angular momentum transport eventually leads to the
collapse of the remnant's core, resulting in a black hole surrounded by a
massive accretion torus. Here we present simulations of this process. The
plausibility of generating short duration gamma ray bursts through this
scenario is discussed.Comment: 3 pages. To appear in the Proceedings of the Eleventh Marcel
Grossmann Meeting, Berlin, Germany, 23-29 July 2006, World Scientific,
Singapore (2007
Black hole-neutron star mergers: effects of the orientation of the black hole spin
The spin of black holes in black hole-neutron star (BHNS) binaries can have a
strong influence on the merger dynamics and the postmerger state; a wide
variety of spin magnitudes and orientations are expected to occur in nature. In
this paper, we report the first simulations in full general relativity of BHNS
mergers with misaligned black hole spin. We vary the spin magnitude from a/m=0
to a/m=0.9 for aligned cases, and we vary the misalignment angle from 0 to 80
degrees for a/m=0.5. We restrict our study to 3:1 mass ratio systems and use a
simple Gamma-law equation of state. We find that the misalignment angle has a
strong effect on the mass of the postmerger accretion disk, but only for angles
greater than ~ 40 degrees. Although the disk mass varies significantly with
spin magnitude and misalignment angle, we find that all disks have very similar
lifetimes ~ 100ms. Their thermal and rotational profiles are also very similar.
For a misaligned merger, the disk is tilted with respect to the final black
hole's spin axis. This will cause the disk to precess, but on a timescale
longer than the accretion time. In all cases, we find promising setups for
gamma-ray burst production: the disks are hot, thick, and hyperaccreting, and a
baryon-clear region exists above the black hole.Comment: 15 pages, 13 figure
Black hole-neutron star mergers for 10 solar mass black holes
General relativistic simulations of black hole-neutron star mergers have
currently been limited to low-mass black holes (less than 7 solar mass), even
though population synthesis models indicate that a majority of mergers might
involve more massive black holes (10 solar mass or more). We present the first
general relativistic simulations of black hole-neutron star mergers with 10
solar mass black holes. For massive black holes, the tidal forces acting on the
neutron star are usually too weak to disrupt the star before it reaches the
innermost stable circular orbit of the black hole. Varying the spin of the
black hole in the range a/M = 0.5-0.9, we find that mergers result in the
disruption of the star and the formation of a massive accretion disk only for
large spins a/M>0.7-0.9. From these results, we obtain updated constraints on
the ability of BHNS mergers to be the progenitors of short gamma-ray bursts as
a function of the mass and spin of the black hole. We also discuss the
dependence of the gravitational wave signal on the black hole parameters, and
provide waveforms and spectra from simulations beginning 7-8 orbits before
merger.Comment: 11 pages, 11 figures - Updated to match published versio
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