470,951 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
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
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
Extracting Energy from Accretion into Kerr Black Hole
The highest efficiency of converting rest mass into energy by accreting
matter into a Kerr black hole is ~ 31% (Thorne 1974). We propose a new process
in which periods of accretion from a thin disk, and the associated spin-up of
the black hole, alternate with the periods of no accretion and magnetic
transfer of energy from the black hole to the disk. These cycles can repeat
indefinitely, at least in principle, with the black hole mass increasing by ~
66% per cycle, and up to ~ 43% of accreted rest mass radiated away by the disk.Comment: 4 pages, 1 figur
Extracting Energy from a Black Hole through Its Disk
When some magnetic field lines connect a Kerr black hole with a disk rotating
around it, energy and angular momentum are transferred between them. If the
black hole rotates faster than the disk, for a thin Keplerian
disk, then energy and angular momentum are extracted from the black hole and
transferred to the disk ( is the mass and is the angular momentum
of the black hole). This way the energy originating in the black hole may be
radiated away by the disk.
The total amount of energy that can be extracted from the black hole spun
down from to by a thin Keplerian disk is
. This is larger than which can be
extracted by the Blandford-Znajek mechanism.Comment: 8 pages, 2 figure
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