2,426 research outputs found
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
Will GRB 990123 Perform an Encore?
The recent gamma ray burst, GRB 990123, has an absorption redshift z_s=1.60,
implying an apparent energy , and a peak luminosity
, assuming isotropic emission. This energy is
ten times larger than hitherto measured and in excess of the rest mass of the
sun. Optical observations have revealed an associated galaxy displaced from the
line of sight by . This raises the possibility that the burst is
enhanced by gravitational lensing. We argue that existing observations probably
only allow magnifications if the galaxy is at z_d=1.60 and the burst
originates at much higher redshift. It should be possible to exclude this
possibility by examining the burst time structure. If, as we anticipate,
multiple imaging can be excluded, GRB 990123 remains the most intrinsically
luminous event yet observed in its entirety.Comment: Accepted by MNRAS: 4 pages, latex, no figure
To the Lighthouse
The extreme hypothesis that essentially all types of ultrarelativistic
outflow -- specifically AGN jets, pulsar wind nebulae and GRB --are
electromagnetic, rather than gas dynamical, phenomena is considered.
Electromagnetic flows are naturally anisotropic and self-collimating so as to
produce jet-like features. The relativistic force-free description of these
flows, which is simpler than the relativistic MHD description, is explained. It
is suggested that the magnetic field associated with AGN jets and GRB is quite
extensively distributed in latitude, without necessarily increasing by much the
total power. It is also proposed that the observed emission from these sources
traces out regions of high current density where global instabilities drive a
turbulence spectrum that is ultimately responsible for the particle
acceleration and the synchrotron, inverse Compton and synchro-Compton emission.
The direct extraction of spin energy from a black hole is re-examined and an
electromagnetic model of GRB explosions is developed. It is also suggested that
some GRB ``lighthouses'' be identified with accretion-induced collapse of a
neutron star to form a black hole in a binary system.Comment: 25 pages, 2 figures. To appear in "Lighthouses of the Universe" Proc.
Symposium held in Garching, Germany (Aug 6-9 2001) ed. M. Gilfanov, R.
Sunyaev et al. Berlin:Springer. Revised version, correcting minor error
Current Issues
Cosmic explosions are observed in many astrophysical environments. They range
in scale from hydromagnetic instabilities in the terrestrial magnetotail and
solar ``nanoflares'' to cosmological gamma ray bursts, supernovae and the
protracted intervals of nuclear activity that produce the giant quasars and
radio galaxies. There are many parallels in the analyses of the explosion sites
that are highlighted at this workshop, specifically stellar coronae, accretion
disks, supernovae and compact objects. In this introductory talk, some general
issues are discussed and some more specific questions relating to the
individual sites are raised.Comment: To appear in Cosmic Explosions: Proc. 10th Maryland Conference on
Astrophysics. Ed. S. Holt and W. Zhang AI
Black Holes and Relativistic Jets
There is strong observational evidence that AGN, Galactic X-ray transients
and (probably) -ray bursts are associated with black holes, and that
these sources are able to form collimated, ultrarelativistic outflows. There is
much interest in trying to understand how these prime movers are able to
release energy from accreting gas and their own spin energy. Electromagnetic
field plays a large role in many of the mechanisms under active consideration.
In this article, several of the many possible ``metabolic pathways'' through
which mass, angular momentum and energy can flow around and away from black
hole magnetospheres are discussed. Particular importance is attached to the
interactions between the inflowing disk, the outflowing wind, the black hole
and the jet. Some important unresolved questions are identified and it is
argued that large scale numerical computation will almost certainly be
necessary to address them.Comment: 21 pages Progress of Theoretical Physics Supplement in pres
Current High Energy Emission from Black Holes
Two related topics are discussed. 1. Accretion onto black holes at low and
high (though not very high) rates is believed to proceed adiabatically ({\em
ie} non-radiatively). It is argued that the liberated energy is carried off by
an outflow, probably involving almost all of the gas that is supplied. Two
dimensional, fluid, accretion disks, in which mass, angular momentum and energy
are transported to the disk surface, are summarized. It is conjectured that
relatively minor changes are needed to describe magnetised disks. By contrast,
the disk surface physics is argued to dictate the character of the outflow. 2.
Ultrarelativistic jets appear to be produced by active galactic nuclei (AGN),
pulsars and -ray bursts (GRB). In all three cases, it is argued that
the power is generated electromagnetically by a magnetic rotator, (in a DC not
AC form), and transported in this manner to the emission site. A model of a
relativistically expanding electromagnetic shell is described and used to
provide a simple model of a GRB in which the -rays are produced by
unstable electrical currents flowing along the rotation axis. The shell drives
a relativistic blast wave into the surrounding medium with a speed that varies
with latitude and whose afterglow emission may exhibit achromatic breaks.
Similar processes may be at work in non-relativistic plerions like the Crab
Nebula and, possibly, AGN jets. The observational implications of these two
classes of model and the prospects for performing instructive, numerical
experiments to elucidate them further are briefly outlined.Comment: 18 pages, 1 figure. To appear in "Current High-Energy Emission around
Black Holes" Proc. 2nd KIAS Astrophysics Workshop held in Seoul, Korea (Sep
3-7 2001) ed. C.-H. Lee Singapore:World Scientifi
The Phenomena of High Energy Astrophysics
A brief summary of some highlights in the study of high energy astrophysical
sources over the past decade is presented. It is argued that the great progress
that has been made derives largely from the application of new technology to
observation throughout all of the electromagnetic and other spectra and that,
on this basis, the next decade should be even more exciting. However, it is
imperative to observe cosmic sources throughout these spectra in order to
obtain a full understanding of their properties. In addition, it is necessary
to learn the universal laws that govern the macroscopic and the microscopic
behavior of cosmic plasma over a great range of physical conditions by
combining observations of different classes of source. These two injunctions
are illustrated by discussions of cosmology, hot gas, supernova remnants and
explosions, neutron stars, black holes and ultrarelativistic outflows. New
interpreations of the acceleration of Galactic cosmic rays, the cooling of hot
gas in rich clusters and the nature of ultrarelativistic outflows are outlined.
The new frontiers of VHE -ray astronomy, low frequency radio astronomy,
neutrino astronomy, UHE cosmic ray physics and gravitational wave astronomy are
especially promising.Comment: To appear in "High Energy Processes and Phenomena in Astrophysics,
IAU Symposium 214", X. Li, Z. Wang, V. Trimble (eds
Origin and Evolution of Massive Black Holes in Galactic Nuclei
Beyond all reasonable doubt, black holes are commonly found in the nuclei of
most normal galaxies. In recent years, dynamical measurements of hole masses
have transformed the study of their functioning and evolution. In particular,
relating their masses, as measured contemporaneously, to the properties of
distant quasars can constrain models of the combined evolution of black holes
and their host galaxies. It is suggested that black hole growth is
radiation-dominated and demand-limited with an e-folding time of Myr
and that most local black hole mass was assembled in AGN with redshifts, z>2,
whose counterparts are not directly observed today. Black hole binaries have
additional features and observable consequences.Comment: 13pp. To appear in Galaxy Dynamics ed. Meritt, Valluri and Sellwood
ASP conf. serie
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