146 research outputs found
Relativistic Accretion
A brief summary of the properties of astrophysical black holes is presented.
Various modes of accretion are distinguished, corresponding to accretion at
rates from well below to well above the Eddington rate. The importance of mass
loss is emphasized when the accreting gas cannot radiate and it is asserted
that a strong wind is likely to be necessary to carry off mass, angular
momentum and energy from the accreting gas. The possible importance of the
black hole spin in the formation of jets and in dictating the relative
importance of non-thermal emission over thermal radiation is discussed.Comment: To appear in "Astrophysical Discs", ASP Conference Series, 13 pages,
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Intrinsic and Extrinsic Galaxy Alignment
We show with analytic models that the assumption of uncorrelated intrinsic
ellipticities of target sources that is usually made in searches for weak
gravitational lensing due to large-scale mass inhomogeneities (``field
lensing'') is unwarranted. If the orientation of the galaxy image is determined
either by the angular momentum or the shape of the halo in which it forms, then
the image should be aligned preferentially with the component of the tidal
gravitational field perpendicular to the line of sight. Long-range correlations
in the tidal field will thus lead to long-range ellipticity-ellipticity
correlations that mimic the shear correlations due to weak gravitational
lensing. We calculate the ellipticity-ellipticity correlation expected if halo
shapes determine the observed galaxy shape, and we discuss uncertainties (which
are still considerable) in the predicted amplitude of this correlation. The
ellipticity-ellipticity correlation induced by angular momenta should be
smaller. We consider several methods for discriminating between the
weak-lensing (extrinsic) and intrinsic correlations, including the use of
redshift information. An ellipticity--tidal-field correlation also implies the
existence of an alignment of images of galaxies near clusters. Although the
intrinsic alignment may complicate the interpretation of field-lensing results,
it is inherently interesting as it may shed light on galaxy formation as well
as on structure formation.Comment: 7 pages, submitted to MNRA
Structure of Magnetocentrifugal Disk-Winds: From the Launching Surface to Large Distances
Protostellar jets and winds are probably driven magnetocentrifugally from the
surface of accretion disks close to the central stellar objects. The exact
launching conditions on the disk, such as the distributions of magnetic flux
and mass ejection rate, are poorly unknown. They could be constrained from
observations at large distances, provided that a robust model is available to
link the observable properties of the jets and winds at the large distances to
the conditions at the base of the flow. We discuss the difficulties in
constructing such large-scale wind models, and describe a novel technique which
enables us to numerically follow the acceleration and propagation of the wind
from the disk surface to arbitrarily large distances and the collimation of
part of the wind into a dense, narrow ``jet'' around the rotation axis. Special
attention is paid to the shape of the jet and its mass flux relative to that of
the whole wind. The mass flux ratio is a measure of the jet formation
efficiency.Comment: 6 pages, figures included. To appear in "The Origins of Stars and
Planets: The VLT View". J. Alves and M. McCaughrean, editor
Black hole magnetosphere with small scale flux tubes--II. Stability and dynamics
In some Seyfert Galaxies, the hard X-rays that produce fluorescent emission
lines are thought to be generated in a hot corona that is compact and located
at only a few gravitational radii above the supermassive black hole. We
consider the possibility that this X-ray source may be powered by small scale
magnetic flux tubes attached to the accretion disk near the black hole. We use
three dimensional, time dependent force-free simulations in a simplified
setting to study the dynamics of such flux tubes as they get continuously
twisted by the central compact star/black hole. We find that, the dynamical
evolution of the flux tubes connecting the central compact object and the
accretion disk is strongly influenced by the confinement of the surrounding
field. Although differential rotation between the central object and the disk
tends to inflate the flux tubes, strong confinement from surrounding field
quenches the formation of a jet-like outflow, as the inflated flux tube becomes
kink unstable and dissipates most of the extracted rotational energy relatively
close to the central object. Such a process may be able to heat up the plasma
and produce strong X-ray emission. We estimate the energy dissipation rate and
discuss its astrophysical implications.Comment: 16 pages, 17 figures. Accepted for publication in MNRA
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