12,279 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
Real Space Effective Interaction and Phase Transition in the Lowest Landau Level
The transition between the stripe state and the liquid state in a high
magnetic field is studied by the density-matrix renormalization-group (DMRG)
method. Systematic analysis on the ground state of two-dimensional electrons in
the lowest Landau level shows that the transition from the stripe state to the
liquid state at v=3/8 is caused by a reduction of repulsive interaction around
r=3. The same reduction of the interaction also stabilizes the incompressible
liquid states at v=1/3 and 2/5, which shows a similarity between the two liquid
states at v=3/8 and 1/3. It is also shown that the strong short-range
interaction around r=1 in the lowest Landau level makes qualitatively different
stripe correlations compared with that in higher Landau levels.Comment: 5 pages, to appear in J. Phys. Soc. Jpn. Vol.73, No.8 (2004
Josephson Plasma in RuSr2GdCu2O8
Josephson plasma in RuSrGdCuO,
RuSrGdCuO (x = 0.3), and
RuSrEuCeCuO (x = 0.5) compounds is
investigated by the sphere resonance method. The Josephson plasma is observed
in a low-frequency region (around 8.5 cm at T ) for
ferromagnetic RuSrGdCuO, while it increases to 35 cm
for non-ferromagnetic RuSrGdCuO (x = 0.3), which
represents a large reduction in the Josephson coupling at ferromagnetic
RuO block layers. The temperature dependence of the plasma does not shift
to zero frequency ({\it i.e.} = 0) at low temperatures, indicating that
there is no transition from the 0-phase to the -phase in these compounds.
The temperature dependence and the oscillator strength of the peak are
different from those of other non-magnetic cuprates, and the origins of these
anomalies are discussed.Comment: to appear in Phys. Rev.B Rapid Com
Black Hole-Neutron Star Mergers: Disk Mass Predictions
Determining the final result of black hole-neutron star mergers, and in
particular the amount of matter remaining outside the black hole at late times
and its properties, has been one of the main motivations behind the numerical
simulation of these systems. Black hole-neutron star binaries are amongst the
most likely progenitors of short gamma-ray bursts --- as long as massive
(probably a few percents of a solar mass), hot accretion disks are formed
around the black hole. Whether this actually happens strongly depends on the
physical characteristics of the system, and in particular on the mass ratio,
the spin of the black hole, and the radius of the neutron star. We present here
a simple two-parameter model, fitted to existing numerical results, for the
determination of the mass remaining outside the black hole a few milliseconds
after a black hole-neutron star merger (i.e. the combined mass of the accretion
disk, the tidal tail, and the potential ejecta). This model predicts the
remnant mass within a few percents of the mass of the neutron star, at least
for remnant masses up to 20% of the neutron star mass. Results across the range
of parameters deemed to be the most likely astrophysically are presented here.
We find that, for 10 solar mass black holes, massive disks are only possible
for large neutron stars (R>12km), or quasi-extremal black hole spins (a/M>0.9).
We also use our model to discuss how the equation of state of the neutron star
affects the final remnant, and the strong influence that this can have on the
rate of short gamma-ray bursts produced by black hole-neutron star mergers.Comment: 11 pages, 7 figure
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Mutational signatures in colon cancer.
ObjectiveRecently, many tumor sequencing studies have inferred and reported on mutational signatures, short nucleotide patterns at which particular somatic base substitutions appear more often. A number of signatures reflect biological processes in the patient and factors associated with cancer risk. Our goal is to infer mutational signatures appearing in colon cancer, a cancer for which environmental risk factors vary by cancer subtype, and compare the signatures to those in adult stem cells from normal colon. We also compare the mutational signatures to others in the literature.ResultsWe apply a probabilistic mutation signature model to somatic mutations previously reported for six adult normal colon stem cells and 431 colon adenocarcinomas. We infer six mutational signatures in colon cancer, four being specific to tumors with hypermutation. Just two signatures explained the majority of mutations in the small number of normal aging colon samples. All six signatures are independently identified in a series of 295 Chinese colorectal cancers
Aligned spin neutron star-black hole mergers: a gravitational waveform amplitude model
The gravitational radiation emitted during the merger of a black hole with a
neutron star is rather similar to the radiation from the merger of two black
holes when the neutron star is not tidally disrupted. When tidal disruption
occurs, gravitational waveforms can be broadly classified in two groups,
depending on the spatial extent of the disrupted material. Extending previous
work by some of us, here we present a phenomenological model for the
gravitational waveform amplitude in the frequency domain encompassing the three
possible outcomes of the merger: no tidal disruption, "mild" and "strong" tidal
disruption. The model is calibrated to 134 general-relativistic numerical
simulations of binaries where the black hole spin is either aligned or
antialigned with the orbital angular momentum. All simulations were produced
using the SACRA code and piecewise polytropic neutron star equations of state.
The present model can be used to determine when black-hole binary waveforms are
sufficient for gravitational-wave detection, to extract information on the
equation of state from future gravitational-wave observations, to obtain more
accurate estimates of black hole-neutron star merger event rates, and to
determine the conditions under which these systems are plausible candidates as
central engines of gamma-ray bursts, macronovae and kilonovae.Comment: 15 pages, 7 figures, 1 tabl
Three-dimensional MHD Simulations of Jets from Accretion Disks
We report the results of 3-dimensional magnetohydrodynamic (MHD) simulations
of a jet formation by the interaction between an accretion disk and a large
scale magnetic field. The disk is not treated as a boundary condition but is
solved self-consistently. To investigate the stability of MHD jet, the
accretion disk is perturbed with a non-axisymmetric sinusoidal or random
fluctuation of the rotational velocity. The dependences of the jet velocity
, mass outflow rate , and mass accretion rate
on the initial magnetic field strength in both non-axisymmetric cases are
similar to those in the axisymmetric case. That is, ,
and where is the
initial magnetic field strength. The former two relations are consistent with
the Michel's steady solution, , although
the jet and accretion do not reach the steady state. In both perturbation
cases, a non-axisymmetric structure with appears in the jet, where
means the azimuthal wave number. This structure can not be explained by
Kelvin-Helmholtz instability and seems to originate in the accretion disk.
Non-axisymmetric modes in the jet reach almost constant levels after about 1.5
orbital periods of the accretion disk, while all modes in the accretion disk
grow with oscillation. As for the angular momentum transport by Maxwell stress,
the vertical component, , in the wide range of initial magnetic field
strength.Comment: Accepted for publication in ApJ. The pdf file with high resolution
figures can be downloaded at
http://www.kusastro.kyoto-u.ac.jp/~hiromitu/3j050806.pd
Gravitational waves from nonspinning black hole-neutron star binaries: dependence on equations of state
We report results of a numerical-relativity simulation for the merger of a
black hole-neutron star binary with a variety of equations of state (EOSs)
modeled by piecewise polytropes. We focus in particular on the dependence of
the gravitational waveform at the merger stage on the EOSs. The initial
conditions are computed in the moving-puncture framework, assuming that the
black hole is nonspinning and the neutron star has an irrotational velocity
field. For a small mass ratio of the binaries (e.g., MBH/MNS = 2 where MBH and
MNS are the masses of the black hole and neutron star, respectively), the
neutron star is tidally disrupted before it is swallowed by the black hole
irrespective of the EOS. Especially for less-compact neutron stars, the tidal
disruption occurs at a more distant orbit. The tidal disruption is reflected in
a cutoff frequency of the gravitational-wave spectrum, above which the spectrum
amplitude exponentially decreases. A clear relation is found between the cutoff
frequency of the gravitational-wave spectrum and the compactness of the neutron
star. This relation also depends weakly on the stiffness of the EOS in the core
region of the neutron star, suggesting that not only the compactness but also
the EOS at high density is reflected in gravitational waveforms. The mass of
the disk formed after the merger shows a similar correlation with the EOS,
whereas the spin of the remnant black hole depends primarily on the mass ratio
of the binary, and only weakly on the EOS. Properties of the remnant disks are
also analyzed.Comment: 27pages, 21 figures; erratum is added on Aug 5. 201
The Acceleration Mechanism of Resistive MHD Jets Launched from Accretion Disks
We analyzed the results of non-linear resistive magnetohydrodynamical (MHD)
simulations of jet formation to study the acceleration mechanism of
axisymmetric, resistive MHD jets. The initial state is a constant angular
momentum, polytropic torus threaded by weak uniform vertical magnetic fields.
The time evolution of the torus is simulated by applying the CIP-MOCCT scheme
extended for resistive MHD equations. We carried out simulations up to 50
rotation period at the innermost radius of the disk created by accretion from
the torus. The acceleration forces and the characteristics of resistive jets
were studied by computing forces acting on Lagrangian test particles. Since the
angle between the rotation axis of the disk and magnetic field lines is smaller
in resistive models than in ideal MHD models, magnetocentrifugal acceleration
is smaller. The effective potential along a magnetic field line has maximum
around in resistive models, where is the radius where the
density of the initial torus is maximum. Jets are launched after the disk
material is lifted to this height by pressure gradient force. Even in this
case, the main acceleration force around the slow magnetosonic point is the
magnetocentrifugal force. The power of the resistive MHD jet is comparable to
the mechanical energy liberated in the disk by mass accretion. Joule heating is
not essential for the formation of jets.Comment: 15 pages, 15 figures, 1 table, accepted for publication in Ap
Magnetically Driven Jets in the Kerr Metric
We compute a series of three-dimensional general relativistic
magnetohydrodynamic simulations of accretion flows in the Kerr metric to
investigate the properties of the unbound outflows that result. The overall
strength of these outflows increases sharply with increasing black hole
rotation rate, but a number of generic features are found in all cases. The
mass in the outflow is concentrated in a hollow cone whose opening angle is
largely determined by the effective potential for matter orbiting with angular
momentum comparable to that of the innermost stable circular orbit. The
dominant force accelerating the matter outward comes from the pressure of the
accretion disk's corona. The principal element that shapes the outflow is
therefore the centrifugal barrier preventing accreting matter from coming close
to the rotation axis. Inside the centrifugal barrier, the cone contains very
little matter and is dominated by electromagnetic fields that rotate at a rate
tied closely to the rotation of the black hole. These fields carry an
outward-going Poynting flux whose immediate energy source is the rotating
spacetime of the Kerr black hole. When the spin parameter a/M of the black hole
exceeds ~0.9, the energy carried to infinity by these outflows can be
comparable to the nominal radiative efficiency predicted in the Novikov-Thorne
model. Similarly, the expelled angular momentum can be comparable to that
accreted by the black hole. Both the inner electromagnetic part and the outer
matter part can contribute in significant fashion to the energy and angular
momentum of the outflow.Comment: 43 pages 12 figures To Appear in the Astrophysical Journal replaced
figure 3c with correct imag
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