951 research outputs found
High-Frequency Voronoi Noise Reduced by Smoothed Mesh Motion
We describe a technique for improving the performance of hydrodynamics codes
which employ a moving Voronoi mesh. Currently, such codes are susceptible to
high-frequency noise produced by rapid adjustments in the grid topology on the
smallest scales. The treatment for this grid noise is simple; instead of moving
the mesh-generating marker points with the local fluid velocity, this velocity
field is smoothed on small scales, so that neighboring marker points generally
have similar velocities. We demonstrate significant improvement gained by this
adjustment in several code tests relevant to the physics which moving-mesh
codes are designed to capture.Comment: MNRAS Accepte
MHD simulations of the collapsar model for GRBs
We present results from axisymmetric, time-dependent magnetohydrodynamic
(MHD) simulations of the collapsar model for gamma-ray bursts. Our main
conclusion is that, within the collapsar model, MHD effects alone are able to
launch, accelerate and sustain a strong polar outflow. We also find that the
outflow is Poynting flux-dominated, and note that this provides favorable
initial conditions for the subsequent production of a baryon-poor fireball.Comment: 4 pages, to appear in proceedings of "2003 GRB Conference" (Santa Fe,
NM, September 8-12, 2003), needs aipprocs LaTeX class, movies are available
at http://rocinante.colorado.edu/~proga
The origin and propagation of variability in the outflows of long duration gamma-ray bursts
We present the results of hydrodynamical simulations of gamma-ray burst jets
propagating through their stellar progenitor material and subsequently through
the surrounding circumstellar medium. We consider both jets that are injected
with constant properties in the center of the star and jets injected with a
variable luminosity. We show that the variability properties of the jet outside
the star are a combination of the variability injected by the engine and the
variability caused by the jet propagation through the star. Comparing power
spectra for the two cases shows that the variability injected by the engine is
preserved even if the jet is heavily shocked inside the star. Such shocking
produces additional variability at long time scales, of order several seconds.
Our findings suggest that the broad pulses of several seconds duration
typically observed in gamma-ray bursts are due to the interaction of the jet
with the progenitor, while the short-timescale variability, characterized by
fluctuations on time scales of milliseconds, has to be injected at the base of
the jet. Studying the properties of the fast variability in GRBs may therefore
provide clues to the nature of the inner engine and the mechanisms of energy
extraction from it.Comment: 23 pages, 13 figures, published in Ap
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