4,203 research outputs found
Fully General Relativistic Simulations of Core-Collapse Supernovae with An Approximate Neutrino Transport
We present results from the first generation of multi-dimensional
hydrodynamic core-collapse simulations in full general relativity (GR) that
include an approximate treatment of neutrino transport. Using a M1 closure
scheme with an analytic variable Eddington factor, we solve the
energy-independent set of radiation energy and momentum based on the Thorne's
momentum formalism. To simplify the source terms of the transport equations, a
methodology of multiflavour neutrino leakage scheme is partly employed. Our
newly developed code is designed to evolve the Einstein field equation together
with the GR radiation hydrodynamic equations. We follow the dynamics starting
from the onset of gravitational core-collapse of a 15 star, through
bounce, up to about 100 ms postbounce in this study to study how the spacial
multi-dimensionality and GR would affect the dynamics in the early postbounce
phase. Our 3D results support the anticipation in previous 1D results that the
neutrino luminosity and average neutrino energy of any neutrino flavor in the
postbounce phase increase when switching from SR to GR hydrodynamics. This is
because the deeper gravitational well of GR produces more compact core
structures, and thus hotter neutrino spheres at smaller radii. By analyzing the
residency timescale to the neutrino-heating timescale in the gain region, we
show that the criterion to initiate neutrino-driven explosions can be most
easily satisfied in 3D models, irrespective of SR or GR hydrodynamics. Our
results suggest that the combination of GR and 3D hydrodynamics provides the
most favorable condition to drive a robust neutrino-driven explosion.Comment: 50pages, 20 figures, Accepted by ApJ. Latest version with following
the referee's suggestions and comment
Specifying the Environments around GRB, Explaining the Fe line in the X-Ray Afterglow of GRB000214
We present a model explaining the Fe K alpha line and the continuum in the
afterglow of GRB000214. We pose the importance to seek the physically natural
environment around GRB000214. For the reproduction of the observation, we need
the ring-like remnant around the progenitor like that of SN 1987A produced by
the mass-loss of the progenitor and the fireball spread over in every
directions. The observation of GRB000214, in which the continuum power law
spectrum decreased faster than the line, motivated us to consider the two
independent systems for the line emission and the continuum spectrum. At first,
the continuum spectrum can be fitted by the afterglow emission of the fireball
pointing toward the observer which does not collide with the ring because the
emission of GRB and the afterglow are highly collimated to the observer by the
relativistic beaming effect. Secondly, the line can be fitted by the
fluorescence of the Fe atoms in the ring illuminated by the X-ray afterglow.
The significance of this study is that our model may constrain strongly the GRB
model. Although the Supranova model assumes the extreme-ring-like remnant
produced by the usual supernova explosion, this may not be probable. It is
because the supernova remnants are known to be shell-like. The model also
assumes two steps of explosions, on the other hand, we need only one explosion
of the progenitor. In this sense, our scenario is more natural. Moreover, in
the numerical simulations of Hypernova, the jet of the opening angle of only 1
degree is generated. In our model, the fireball which spreads over in every
directions reconciles with the observation of 1 percent of the polarization in
the observation of SN1998bw which showed the explosion might not be so
collimated.Comment: 26 pages and 2 postscript figures. to appear in Publications of the
Astronomical Society of Japan. In this revision, we added some discussions
and changed several English expresson
Three-dimensional Hydrodynamic Core-Collapse Supernova Simulations for an Star with Spectral Neutrino Transport
We present numerical results on three-dimensional (3D) hydrodynamic
core-collapse simulations of an star. By comparing one-(1D)
and two-dimensional(2D) results with those of 3D, we study how the increasing
spacial multi-dimensionality affects the postbounce supernova dynamics. The
calculations were performed with an energy-dependent treatment of the neutrino
transport that is solved by the isotropic diffusion source approximation
scheme. By performing a tracer-particle analysis, we show that the maximum
residency time of material in the gain region is shown to be longer for 3D due
to non-axisymmetric flow motions than 2D, which is one of advantageous aspects
of 3D models to obtain neutrino-driven explosions. Our results show that
convective matter motions below the gain radius become much more violent in 3D
than 2D, making the neutrino luminosity larger for 3D. Nevertheless the emitted
neutrino energies are made smaller due to the enhanced cooling. Our results
indicate whether these advantages for driving 3D explosions could or could not
overwhelm the disadvantages is sensitive to the employed numerical resolutions.
An encouraging finding is that the shock expansion tends to become more
energetic for models with finer resolutions. To draw a robust conclusion, 3D
simulations with much more higher numerical resolutions and also with more
advanced treatment of neutrino transport as well as of gravity is needed, which
could be hopefully practicable by utilizing forthcoming Petaflops-class
supercomputers.Comment: 16 pages, 18 figures, accepted for publication in Ap
Gravitational Wave Signatures of Hyperaccreting Collapsar Disks
By performing two-dimensional special relativistic (SR) magnetohydrodynamic
simulations, we study possible signatures of gravitational waves (GWs) in the
context of the collapsar model for long-duration gamma-ray bursts. In our SR
simulations, the central black hole is treated as an absorbing boundary. By
doing so, we focus on the GWs generated by asphericities in neutrino emission
and matter motions in the vicinity of the hyperaccreting disks. We compute nine
models by adding initial angular momenta and magnetic fields parametrically to
a precollapse core of a progenitor star. As for the
microphysics, a realistic equation of state is employed and the neutrino
cooling is taken into account via a multiflavor neutrino leakage scheme. To
accurately estimate GWs produced by anisotropic neutrino emission, we perform a
ray-tracing analysis in general relativity by a post-processing procedure. By
employing a stress formula that includes contributions both from magnetic
fields and special relativistic corrections, we study also the effects of
magnetic fields on the gravitational waveforms. We find that the GW amplitudes
from anisotropic neutrino emission show a monotonic increase with time, whose
amplitudes are much larger than those from matter motions of the accreting
material. We show that the increasing trend of the neutrino GWs stems from the
excess of neutrino emission in the direction near parallel to the spin axis
illuminated from the hyperaccreting disks. We point out that a recently
proposed future space-based interferometer like Fabry-Perot type DECIGO would
permit the detection of these GW signals within 100 Mpc.Comment: 38 pages, 14 figures, ApJ in pres
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