2,817 research outputs found
Why Is Supercritical Disk Accretion Feasible?
Although the occurrence of steady supercritical disk accretion onto a black
hole has been speculated about since the 1970s, it has not been accurately
verified so far. For the first time, we previously demonstrated it through
two-dimensional, long-term radiation-hydrodynamic simulations. To clarify why
this accretion is possible, we quantitatively investigate the dynamics of a
simulated supercritical accretion flow with a mass accretion rate of ~10^2
L_E/c^2 (with L_E and c being, respectively, the Eddington luminosity and the
speed of light). We confirm two important mechanisms underlying supercritical
disk accretion flow, as previously claimed, one of which is the radiation
anisotropy arising from the anisotropic density distribution of very optically
thick material. We qualitatively show that despite a very large radiation
energy density, E_0>10^2L_E/(4 pi r^2 c) (with r being the distance from the
black hole), the radiative flux F_0 cE_0/tau could be small due to a large
optical depth, typically tau 10^3, in the disk. Another mechanism is photon
trapping, quantified by vE_0, where v is the flow velocity. With a large |v|
and E_0, this term significantly reduces the radiative flux and even makes it
negative (inward) at r<70r_S, where r_S is the Schwarzschild radius. Due to the
combination of these effects, the radiative force in the direction along the
disk plane is largely attenuated so that the gravitational force barely exceeds
the sum of the radiative force and the centrifugal force. As a result, matter
can slowly fall onto the central black hole mainly along the disk plane with
velocity much less than the free-fall velocity, even though the disk luminosity
exceeds the Eddington luminosity. Along the disk rotation axis, in contrast,
the strong radiative force drives strong gas outflows.Comment: 8 pages, 7 figures, accepted for publication in Ap
Super-critical Accretion Flows around Black Holes: Two-dimensional, Radiation-pressure-dominated Disks with Photon-trapping
The quasi-steady structure of super-critical accretion flows around a black
hole is studied based on the two-dimensional radiation-hydrodynamical (2D-RHD)
simulations. The super-critical flow is composed of two parts: the disk region
and the outflow regions above and below the disk. Within the disk region the
circular motion as well as the patchy density structure are observed, which is
caused by Kelvin-Helmholtz instability and probably by convection. The
mass-accretion rate decreases inward, roughly in proportion to the radius, and
the remaining part of the disk material leaves the disk to form outflow because
of strong radiation pressure force. We confirm that photon trapping plays an
important role within the disk. Thus, matter can fall onto the black hole at a
rate exceeding the Eddington rate. The emission is highly anisotropic and
moderately collimated so that the apparent luminosity can exceed the Eddington
luminosity by a factor of a few in the face-on view. The mass-accretion rate
onto the black hole increases with increase of the absorption opacity
(metalicity) of the accreting matter. This implies that the black hole tends to
grow up faster in the metal rich regions as in starburst galaxies or
star-forming regions.Comment: 16 pages, 12 figures, accepted for publication in ApJ (Volume 628,
July 20, 2005 issue
Heavy Higgs at Tevatron and LHC in Universal Extra Dimension Models
Universal Extra Dimension (UED) models tend to favor a distinctively heavier
Higgs mass than in the Standard Model (SM) and its supersymmetric extensions
when the Kaluza-Klein (KK) scale is not much higher than the electroweak one,
which we call the weak scale UED, in order to cancel the KK top contributions
to the T-parameter. Such a heavy Higgs, whose production through the gluon
fusion process is enhanced by the KK top loops, is fairly model independent
prediction of the weak scale UED models regardless of the brane-localized mass
structure at the ultraviolet cutoff scale. We study its cleanest possible
signature, the Higgs decay into a Z boson pair and subsequently into four
electrons and/or muons, in which all the four-momenta of the final states can
be measured and both the Z boson masses can be checked. We show that the weak
scale UED model may account for the 2sigma excess of this event at ATLAS at the
ZZ pair invariant mass around 250GeV, at which scale SM background is
sufficiently small and the SM Higgs predicts too few events. We have also
studied the Higgs mass 500GeV (and also 700GeV with \sqrt{s}=14TeV) and have
found that we can observe significant resonance with the integrated luminosity
10fb^{-1} for six dimensional UED models.Comment: (v1) 36 pages, 9 figures, 6 tables; (v2) Accepted for publication in
Phys. Rev. D, factor 2 error in (93) corrected, comments and references
added, figures redrawn; (v3) Minor changes including typo corrections in
eq.(15), final version appearing in PR
Dissipation Instability in the Accretion Disk
The model of a geometrically thin gaseous disk in the external gravitational
potential is considered. The dinamics of small nonaxisymmetric perturbations in
the plane of the accretion disk with dissipative effects is investigated. It is
showed, that conditions of development and parameters of unstable oscillation
modes in the opticaly thick accretion disk are strongly depended on the models
of viscosity and opacity.Comment: Plain TeX, 6 pages, 2 figures (GIF), Submitted to Astron. Astrophys.
Transaction
Anisotropic magnetic properties of CeAgGe single crystal
In order to investigate the anisotropic magnetic properties of
CeAgGe, we have successfully grown the single crystals, for the first
time, by high temperature solution growth (flux) method. We have performed a
detailed study of the grown single crystals by measuring their electrical
resistivity, magnetic susceptibility, magnetization, specific heat and
magnetoresistance. A clear anisotropy and an antiferromagnetic transition at
= 4.6 K have been observed in the magnetic properties. The magnetic
entropy reaches ln 4 at 20 K indicating that the ground state and the first
excited state are very closely spaced (a quasi-quartet state). From the
specific heat measurements and crystalline electric field (CEF) analysis of the
magnetic susceptibility, we have found the level splitting energies as 5 K and
130 K. The magnetization measurements reveal that the a-axis is the easy axis
of magnetization and the saturation moment is = 1.6 /Ce, corroborating the previous neutron diffraction measurements on a
polycrystalline sample.Comment: Submitted to Phys. Rev.
A prediction for bubbling geometries
We study the supersymmetric circular Wilson loops in N=4 Yang-Mills theory.
Their vacuum expectation values are computed in the parameter region that
admits smooth bubbling geometry duals. The results are a prediction for the
supergravity action evaluated on the bubbling geometries for Wilson loops.Comment: 21 pages, latex; v.2 reference added; v.3 minor correction
Theory of Low Temperature Electron Spin Resonance in Half-integer Spin Antiferromagnetic Chains
A theory of low temperature (T) electron spin resonance (ESR) in half-integer
spin antiferromagnetic chains is developed using field theory methods and
avoiding previous approximations. It is compared to experiments on Cu benzoate.
Power laws are predicted for the line-width broadening due to various types of
anisotropy. At T -> 0, zero width absorption peaks occur in some cases. The
second ESR peak in Cu benzoate, observed at T<.76K, is argued not to indicate
Neel order as previously claimed, but to correspond to a sine-Gordon "breather"
excitation.Comment: 4 pages, REVTEX, 3 PostScript figures embedded in tex
A coupled chaotic system with different time scales: Toward the implication of observation with dynamical systems
We present three curious phenomena in coupled chaotic systems with different time scales: a copy, an itinerant motion, and 'firework'. The phenomena obtained may provide possible implication of interacting two macroscopic systems, namely 'observing' and 'observed' systems
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