1,247 research outputs found
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
Gravitational waves from supernova matter
We have performed a set of 11 three-dimensional magnetohydrodynamical core
collapse supernova simulations in order to investigate the dependencies of the
gravitational wave signal on the progenitor's initial conditions. We study the
effects of the initial central angular velocity and different variants of
neutrino transport. Our models are started up from a 15 solar mass progenitor
and incorporate an effective general relativistic gravitational potential and a
finite temperature nuclear equation of state. Furthermore, the electron flavour
neutrino transport is tracked by efficient algorithms for the radiative
transfer of massless fermions. We find that non- and slowly rotating models
show gravitational wave emission due to prompt- and lepton driven convection
that reveals details about the hydrodynamical state of the fluid inside the
protoneutron stars. Furthermore we show that protoneutron stars can become
dynamically unstable to rotational instabilities at T/|W| values as low as ~2 %
at core bounce. We point out that the inclusion of deleptonization during the
postbounce phase is very important for the quantitative GW prediction, as it
enhances the absolute values of the gravitational wave trains up to a factor of
ten with respect to a lepton-conserving treatment.Comment: 10 pages, 6 figures, accepted, to be published in a Classical and
Quantum Gravity special issue for MICRA200
The Core-Collapse Supernova with "Non-Uniform" Magnetic Fields
We perform two-dimensional numerical simulations on the core-collapse of a
massive star with strong magnetic fields and differential rotations using a
numerical code ZEUS-2D. Changing field configurations and laws of differential
rotation parametrically, we compute 14 models and investigate effects of these
parameters on the dynamics. In our models, we do not solve the neutrino
transport and instead employ a phenomenological parametric EOS that takes into
account the neutrino emissions. As a result of the calculations, we find that
the field configuration plays a significant role in the dynamics of the core if
the initial magnetic field is large enough. Models with initially concentrated
fields produce more energetic explosions and more prolate shock waves than the
uniform field. Quadrapole-like fields produce remarkably collimated and fast
jet, which might be important for gamma-ray bursts(GRB). The Lorentz forces
exerted in the region where the plasma-beta is less than unity are responsible
for these dynamics. The pure toroidal field, on the other hand, does not lead
to any explosion or matter ejection. This suggests the presupernova models of
Heger et al.(2003), in which toroidal fields are predominant, is
disadvantageous for the magnetorotation-induced supernova considered here.
Models with initially weak magnetic fields do not lead to explosion or matter
ejection, either. In these models magnetic fields play no role as they do not
grow on the timescale considered in this paper so that the magnetic pressure
could be comparable to the matter pressure. This is because the exponential
field growth as expected in MRI is not seen in our models. The magnetic field
is amplified mainly by field-compression and field-wrapping in our simulations.Comment: 24 pages, 5 figures, ApJ in press, typos correcte
Magnetohydrodynamic Simulations of A Rotating Massive Star Collapsing to A Black Hole
We perform two-dimensional, axisymmetric, magnetohydrodynamic simulations of
the collapse of a rotating star of 40 Msun and in the light of the collapsar
model of gamma-ray burst. Considering two distributions of angular momentum, up
to \sim 10^{17} cm^2/s, and the uniform vertical magnetic field, we investigate
the formation of an accretion disk around a black hole and the jet production
near the hole. After material reaches to the black hole with the high angular
momentum, the disk is formed inside a surface of weak shock. The disk becomes
in a quasi-steady state for stars whose magnetic field is less than 10^{10} G
before the collapse. We find that the jet can be driven by the magnetic fields
even if the central core does not rotate as rapidly as previously assumed and
outer layers of the star has sufficiently high angular momentum. The magnetic
fields are chiefly amplified inside the disk due to the compression and the
wrapping of the field. The fields inside the disk propagate to the polar region
along the inner boundary near the black hole through the Alfv{\'e}n wave, and
eventually drive the jet. The quasi-steady disk is not an advection-dominated
disk but a neutrino cooling-dominated one. Mass accretion rates in the disks
are greater than 0.01 Msun/sec with large fluctuations. The disk is transparent
for neutrinos. The dense part of the disk, which locates near the hole, emits
neutrino efficiently at a constant rate of < 8 \times 10^{51} erg/s. The
neutrino luminosity is much smaller than those from supernovae after the
neutrino burst.Comment: 42 pages, accepted for publication in the Astrophysical Journal. A
paper with higher-resolution figures available at
http://www.ec.knct.ac.jp/~fujimoto/collapsar/mhd-color.pd
Numerical Simulations of Equatorially-Asymmetric Magnetized Supernovae: Formation of Magnetars and Their Kicks
A series of numerical simulations on magnetorotational core-collapse
supernovae are carried out. Dipole-like configurations which are offset
northward are assumed for the initially strong magnetic fields together with
rapid differential rotations. Aims of our study are to investigate effects of
the offset magnetic field on magnetar kicks and on supernova dynamics. Note
that we study a regime where the proto-neutron star formed after collapse has a
large magnetic field strength approaching that of a ``magnetar'', a highly
magnetized slowly rotating neutron star. As a result, equatorially-asymmetric
explosions occur with a formation of the bipolar jets. Resultant magnetar's
kick velocities are km s. We find that the acceleration
is mainly due to the magnetic pressure while the somewhat weaker magnetic
tension works toward the opposite direction, which is due to stronger magnetic
field in the northern hemisphere. Noted that observations of magnetar's proper
motions are very scarce, our results supply a prediction for future
observations. Namely, magnetars possibly have large kick velocities, several
hundred km s, as ordinary neutron stars do, and in an extreme case they
could have those up to 1000 km s.Comment: 36 pages, 9 figures, accepted by the Astrophysical Journa
Neutrino oscillations in magnetically driven supernova explosions
We investigate neutrino oscillations from core-collapse supernovae that
produce magnetohydrodynamic (MHD) explosions. By calculating numerically the
flavor conversion of neutrinos in the highly non-spherical envelope, we study
how the explosion anisotropy has impacts on the emergent neutrino spectra
through the Mikheyev-Smirnov-Wolfenstein effect. In the case of the inverted
mass hierarchy with a relatively large theta_(13), we show that survival
probabilities of electron type neutrinos and antineutrinos seen from the
rotational axis of the MHD supernovae (i.e., polar direction), can be
significantly different from those along the equatorial direction. The event
numbers of electron type antineutrinos observed from the polar direction are
predicted to show steepest decrease, reflecting the passage of the
magneto-driven shock to the so-called high-resonance regions. Furthermore we
point out that such a shock effect, depending on the original neutrino spectra,
appears also for the low-resonance regions, which leads to a noticeable
decrease in the electron type neutrino signals. This reflects a unique nature
of the magnetic explosion featuring a very early shock-arrival to the resonance
regions, which is in sharp contrast to the neutrino-driven delayed supernova
models. Our results suggest that the two features in the electron type
antineutrinos and neutrinos signals, if visible to the Super-Kamiokande for a
Galactic supernova, could mark an observational signature of the magnetically
driven explosions, presumably linked to the formation of magnetars and/or
long-duration gamma-ray bursts.Comment: 25 pages, 21 figures, JCAP in pres
Effects of QCD phase transition on gravitational radiation from two-dimensional collapse and bounce of massive stars
We perform two-dimensional, magnetohydrodynamical core-collapse simulations
of massive stars accompanying the QCD phase transition. We study how the phase
transition affects the gravitational waveforms near the epoch of core-bounce.
As for initial models, we change the strength of rotation and magnetic fields.
Particularly, the degree of differential rotation in the iron core (Fe-core) is
changed parametrically. As for the microphysics, we adopt a phenomenological
equation of state above the nuclear density, including two parameters to change
the hardness before the transition. We assume the first order phase transition,
where the conversion of bulk nuclear matter to a chirally symmetric quark-gluon
phase is described by the MIT bag model. Based on these computations, we find
that the phase transition can make the maximum amplitudes larger up to
10 percents than the ones without the phase transition. On the other hand, the
maximum amplitudes become smaller up to 10 percents owing to the phase
transition, when the degree of the differential rotation becomes larger. We
find that even extremely strong magnetic fields G in the
protoneutron star do not affect these results.Comment: 12 pages, 12 figures. Resubmitted to Phys.Rev.
Gravitational Waves from Core Collapse Supernovae
We present the gravitational wave signatures for a suite of axisymmetric core
collapse supernova models with progenitors masses between 12 and 25 solar
masses. These models are distinguished by the fact they explode and contain
essential physics (in particular, multi-frequency neutrino transport and
general relativity) needed for a more realistic description. Thus, we are able
to compute complete waveforms (i.e., through explosion) based on
non-parameterized, first-principles models. This is essential if the waveform
amplitudes and time scales are to be computed more precisely. Fourier
decomposition shows that the gravitational wave signals we predict should be
observable by AdvLIGO across the range of progenitors considered here. The
fundamental limitation of these models is in their imposition of axisymmetry.
Further progress will require counterpart three-dimensional models.Comment: 10 pages, 5 figure
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