70 research outputs found

### 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 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

### Unequal mass binary neutron star simulations with neutrino transport: Ejecta and neutrino emission

We present 12 new simulations of unequal mass neutron star mergers. The simulations are performed with the SpEC code, and utilize nuclear-theory-based equations of state and a two-moment gray neutrino transport scheme with an improved energy estimate based on evolving the number density. We model the neutron stars with the SFHo, LS220, and DD2 equations of state (EOS) and we study the neutrino and matter emission of all 12 models to search for robust trends between binary parameters and emission characteristics. We find that the total mass of the dynamical ejecta exceeds 0.01â€‰â€‰MâŠ™ only for SFHo with weak dependence on the mass ratio across all models. We find that the ejecta have a broad electron fraction (Y_e) distribution (â‰ˆ0.06â€“0.48), with mean 0.2. Y_e increases with neutrino irradiation over time, but decreases with increasing binary asymmetry. We also find that the models have ejecta with a broad asymptotic velocity distribution (â‰ˆ0.05â€“0.7c). The average velocity lies in the range 0.2câˆ’0.3c and decreases with binary asymmetry. Furthermore, we find that disk mass increases with binary asymmetry and stiffness of the EOS. The Y_e of the disk increases with softness of the EOS. The strongest neutrino emission occurs for the models with soft EOS. For (anti) electron neutrinos we find no significant dependence of the magnitude or angular distribution or neutrino luminosity with mass ratio. The heavier neutrino species have a luminosity dependence on mass ratio but an angular distribution which does not change with mass ratio

### Magnetic effects on the low-T/|W| instability in differentially rotating neutron stars

Dynamical instabilities in protoneutron stars may produce gravitational waves
whose observation could shed light on the physics of core-collapse supernovae.
When born with sufficient differential rotation, these stars are susceptible to
a shear instability (the "low-T/|W| instability"), but such rotation can also
amplify magnetic fields to strengths where they have a considerable impact on
the dynamics of the stellar matter. Using a new magnetohydrodynamics module for
the Spectral Einstein Code, we have simulated a differentially-rotating neutron
star in full 3D to study the effects of magnetic fields on this instability.
Though strong toroidal fields were predicted to suppress the low-T/|W|
instability, we find that they do so only in a small range of field strengths.
Below 4e13 G, poloidal seed fields do not wind up fast enough to have an effect
before the instability saturates, while above 5e14 G, magnetic instabilities
can actually amplify a global quadrupole mode (this threshold may be even lower
in reality, as small-scale magnetic instabilities remain difficult to resolve
numerically). Thus, the prospects for observing gravitational waves from such
systems are not in fact diminished over most of the magnetic parameter space.
Additionally, we report that the detailed development of the low-T/|W|
instability, including its growth rate, depends strongly on the particular
numerical methods used. The high-order methods we employ suggest that growth
might be considerably slower than found in some previous simulations.Comment: REVTeX 4.1, 21 pages, 18 figures, submitting to Physical Review

### Relativistic Magnetohydrodynamics In Dynamical Spacetimes: Numerical Methods And Tests

Many problems at the forefront of theoretical astrophysics require the
treatment of magnetized fluids in dynamical, strongly curved spacetimes. Such
problems include the origin of gamma-ray bursts, magnetic braking of
differential rotation in nascent neutron stars arising from stellar core
collapse or binary neutron star merger, the formation of jets and magnetized
disks around newborn black holes, etc. To model these phenomena, all of which
involve both general relativity (GR) and magnetohydrodynamics (MHD), we have
developed a GRMHD code capable of evolving MHD fluids in dynamical spacetimes.
Our code solves the Einstein-Maxwell-MHD system of coupled equations in
axisymmetry and in full 3+1 dimensions. We evolve the metric by integrating the
BSSN equations, and use a conservative, shock-capturing scheme to evolve the
MHD equations. Our code gives accurate results in standard MHD code-test
problems, including magnetized shocks and magnetized Bondi flow. To test our
code's ability to evolve the MHD equations in a dynamical spacetime, we study
the perturbations of a homogeneous, magnetized fluid excited by a gravitational
plane wave, and we find good agreement between the analytic and numerical
solutions.Comment: 22 pages, 15 figures, accepted for publication in Phys. Rev.

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