472,635 research outputs found
Sino the Times: three spoken drama productions on the Beijing stage
Today's modern theatre in Beijing shows new talents and directions as well as problems that are part of the uncertainties of Chinese society — in what may be the most intriguing transitional period in Chinese history
Making Clean Energy with a Kerr Black Hole: a Tokamak Model for Gamma-Ray Bursts
In this paper we present a model for making clean energy with a Kerr black
hole. Consider a Kerr black hole with a dense plasma torus spinning around it.
A toroidal electric current flows on the surface of the torus, which generates
a poloidal magnetic field outside the torus. On the surface of the tours the
magnetic field is parallel to the surface. The closed magnetic field lines
winding around the torus compress and confine the plasma in the torus, as in
the case of tokamaks. Though it is unclear if such a model is stable, we look
into the consequences if the model is stable. If the magnetic field is strong
enough, the baryonic contamination from the plasma in the torus is greatly
suppressed by the magnetic confinement and a clean magnetosphere of
electron-positron pairs is built up around the black hole. Since there are no
open magnetic field lines threading the torus and no accretion, the power of
the torus is zero. If some magnetic field lines threading the black hole are
open and connect with loads, clean energy can be extracted from the Kerr black
hole by the Blandford-Znajek mechanism.
The model may be relevant to gamma-ray bursts. The energy in the Poynting
flux produced by the Blandford-Znajek mechanism is converted into the kinetic
energy of the electron-positron pairs in the magnetosphere around the black
hole, which generates two oppositely directed jets of electron-positron pairs
with super-high bulk Lorentz factors. The jets collide and interact with the
interstellar medium, which may produce gamma-ray bursts and the afterglows.Comment: 14 pages, 1 figure, accepted by Ap
A Combinatorial Formula for Certain Elements of Upper Cluster Algebras
We develop an elementary formula for certain non-trivial elements of upper
cluster algebras. These elements have positive coefficients. We show that when
the cluster algebra is acyclic these elements form a basis. Using this formula,
we show that each non-acyclic skew-symmetric cluster algebra of rank 3 is
properly contained in its upper cluster algebra
Absolutely Continuous Representations and a Kaplansky Density Theorem for Free Semigroup Algebras
We introduce notions of absolutely continuous functionals and representations
on the non-commutative disk algebra . Absolutely continuous functionals
are used to help identify the type L part of the free semigroup algebra
associated to a -extendible representation . A -extendible
representation of is ``regular'' if the absolutely continuous part
coincides with the type L part. All known examples are regular. Absolutely
continuous functionals are intimately related to maps which intertwine a given
-extendible representation with the left regular representation. A simple
application of these ideas extends reflexivity and hyper-reflexivity results.
Moreover the use of absolute continuity is a crucial device for establishing a
density theorem which states that the unit ball of is weak-
dense in the unit ball of the associated free semigroup algebra if and only if
is regular. We provide some explicit constructions related to the
density theorem for specific representations. A notion of singular functionals
is also defined, and every functional decomposes in a canonical way into the
sum of its absolutely continuous and singular parts.Comment: 26 pages, prepared with LATeX2e, submitted to Journal of Functional
Analysi
Magneto-Centrifugal Launching of Jets from Accretion Disks. I: Cold Axisymmetric Flows
The magneto-centrifugal model for jet formation is studied by time-dependent
simulations reaching steady state in a cold gas with negligible fluid pressure,
in an axisymmetric geometry, using a modification of the Zeus3D code adapted to
parallel computers. The number of boundary conditions imposed at the coronal
base takes into account the existence of the fast and Alfvenic critical
surfaces, avoiding over-determination of the flow. The size and shape of the
computational box is chosen to include these critical surfaces, reducing the
influence of the outer boundary conditions. As there is a region, near the
origin, where the inclination of field lines to the axis is too small to drive
a centrifugal wind, we inject a thin, axial jet, expected to form
electromagnetically near black holes. Acceleration and collimation appear for
wide generic conditions. A reference run is shown in detail, with a wind
leaving the computational volume in the axial direction with a poloidal
velocity equal to 4 times the poloidal Alfven speed, collimated inside 11
degrees. Finally, the critical surfaces, fieldlines, thrust, energy, torque and
mass discharge of the outgoing wind are shown for simulations with various
profiles of mass and magnetic flux at the base of the corona.Comment: 27 pages, including 10 figures and 2 tables. To appear in ApJ (Dec
1999). Revised version clarifies the abstract, section 3.2.4, conclusions and
appendix, adds a simulation to section 4.2, and updates the reference
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