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- The
Introduction
to Accretion
Disks in
Astrophysics: (11 March
2005)
Source: (11 March 2005) - Neutrino-Domin
ated Accretion
Models for
Gamma-Ray
Bursts:
Effects of
General
Relativity and
Neutrino
Opacity: (18 Apr
2006)We first
refine the
fixed concept
in the
literature
that the usage
of the
Newtonian
potential in
studies of
black hole
accretion is
invalid and
the general
relativistic
effect must be
considered. As
our main
results, we
then show that
the energy
released by
neutrino
annihilation
in
neutrino-domin
ated accretion
flows is
sufficient for
gamma-ray
bursts when
the
contribution
from the
optically
thick region
of the flow is
included, and
that in the
optically
thick region
advection does
not
necessarily
dominate over
neutrino
cooling
because the
advection
factor is
relevant to
the
geometrical
depth rather
than the
optical depth
of the flow.
Source: (18 Apr 2006) - A simple
accretion
model of a
rotating gas
sphere onto a
Schwarzschild
black hole: (12 Mar
2006)We
construct a
simple
accretion
model of a
rotating
pressureless
gas sphere
onto a
Schwarzschild
black hole. We
show how to
build analytic
solutions in
terms of
Jacobi
elliptic
functions.
This
construction
represents a
general
relativistic
generalisation
of the
Newtonian
accretion
model first
proposed by
Ulrich (1976).
In exactly the
same form as
it occurs for
the Newtonian
case, the flow
naturally
predicts the
existence of
an equatorial
rotating
accretion disc
about the
hole. However,
the radius of
the disc
increases
monotonically
without limit
as the flow
reaches its
maximum
allowed
angular
momentum.
Source: (12 Mar 2006) - Black-Hole
Accretion Disc
as an Analogue
Gravity Model: (21 Apr
2006)We
formulate and
solve the
equations
governing the
transonic
behaviour of a
general
relativistic
black-hole
accretion disc
with non-zero
advection
velocity. We
demonstrate
that a
relativistic
Rankine-Hugoni
ot shock may
form leading
to the
formation of
accretion
powered
outflow. We
show that the
critical
points of
transonic
discs
generally do
not coincide
with the
corresponding
sonic points.
The collection
of such sonic
points forms
an
axisymmetric
hypersurface,
generators of
which are the
acoustic null
geodesics,
i.e. the
phonon
trajectories.
Such a surface
is shown to be
identical with
an acoustic
event horizon.
The acoustic
surface
gravity and
the
corresponding
analogue
horizon
temperature
$T_AH$ at the
acoustic
horizon are
then computed
in terms of
fundamental
accretion
parameters.
Physically,
the analogue
temperature is
associated
with the
thermal phonon
radiation
analogous to
the Hawking
radiation of
the black-hole
horizon.Thus,
an
axisymmetric
black-hole
accretion disc
is established
as a natural
example of the
classical
analogue
gravity model,
for which two
kinds of
horizon exist
simultaneously
. We have
shown that for
some values of
astrophysicall
y relevant
accretion
parameters,
the analogue
temperature
exceeds the
corresponding
Hawking
temperature.
We point out
that acoustic
can also be
generated for
a
multi-transoni
c black-hole
accretion with
a shock. Such
a white hole,
produced at
the shock, is
always flanked
by two
acoustic black
holes
generated at
the inner and
the outer
sonic points.
Finally, we
discuss
possible
applications
of our work to
other
astrophysical
events which
may exhibit
analogue
effects.
Source: (21 Apr 2006) - Shapes and
Positions of
Black Hole
Shadows in
Accretion
Disks and Spin
Parameters of
Black Holes: (6 May
2004)Can we
determine a
spin parameter
of a black
hole by
observation of
a black hole
shadow in an
accretion
disk? In order
to answer this
question, we
make a
qualitative
analysis and a
quantitative
analysis of a
shape and a
position of a
black hole
shadow casted
by a rotating
black hole on
an optically
thick
accretion disk
and its
dependence on
an angular
momentum of a
black hole. We
have found
black hole
shadows with a
quite similar
size and a
shape for
largely
different
black hole
spin
parameters and
a same black
hole mass.
Thus, it is
practically
difficult to
determine a
spin parameter
of a black
hole from a
size and a
shape of a
black hole
shadow in an
accretion
disk. We newly
introduce a
bisector axis
of a black
hole shadow
named a shadow
axis. For a
rotating black
hole a shape
and a position
of a black
hole shadow
are not
symmetric with
respect to a
rotation axis
of a black
hole shadow.
So, in this
case the
minimum
interval
between a mass
center of a
black hole and
a shadow axis
is finite. An
extent of this
minimum
interval is
roughly
proportional
to a spin
parameter of a
black hole for
a fixed
inclination
angle between
a rotation
axis of a
black hole and
a direction of
an observer.
In order to
measure a spin
parameter of a
black hole, if
a shadow axis
is determined
observationall
y, it is
crucially
important to
determine a
position of a
mass center of
a black hole
in a region of
a black hole
shadow.
Source: (6 May 2004) - Black Hole
Accretion
Disks: (01 August
1998)
Source: (01 August 1998) - Spatio-tempora
l boundary
formation: the
role of local
motion signals
in boundary
perception: Vision
Research, Vol.
37, No. 10.
(14 April
1997), pp.
1281-1293.Spat
io-temporal
boundary
formation
(SBF) refers
to a
perceptual
process
responsible
for perception
of moving,
bounded
surfaces from
sequential
changes in
spatially
separated
local
elements.
Previous
research has
indicated that
this process
produces
perception of
global form,
continuous
boundaries and
global motion
from spatially
and temporally
sparse element
changes. In
the present
paper, we
sought to
distinguish
between two
classes of
models for
SBF:
form-precedes-
motion and
motion-precede
s-form models.
Experiment 1
tested the
effects of the
addition of
spurious
motion
signals, a
manipulation
that should
affect a
motion-precede
s-form
computation
but not a
form-precedes-
motion
computation.
Shape
identification
in a
10-alternative
forced-choice
procedure was
disrupted by
this
manipulation,
supporting the
former class
of models. A
particular
computational
scheme, edge
orientation
from motion
(EOFM)
instantiating
a
motion-precede
s-form model
is described
and tested in
Experiment 2.
The EOFM model
should be
disrupted when
initiating
element
changes occur
in a certain
type of
sequential
order,
relative to
randomly
arranged
changes.
Sequential
changes
markedly
disrupted
performance,
supporting
this EOFM
approach. The
results favor
motion-precede
s-form models
of SBF and are
consistent
with the
particular
computational
scheme
proposed.
Source: Vision Research, Vol. 37, No. 10. (14 April 1997), pp. 1281-1293. - Spatiotemporal
boundary
formation:
boundary,
form, and
motion
perception
from
transformation
s of surface
elements.: Journal of
experimental
psychology.
General, Vol.
123, No. 1.
(March 1994),
pp.
3-20.Continuou
s surface
boundaries,
object shape,
and global
motion can be
perceived from
information
that is
fragmentary in
both space and
time. The
authors report
investigations
indicating
that accretion
and deletion
of texture is
only 1 member
of a broader
class of
element
transformation
s that produce
boundaries,
shape, and
motion,
through
spatiotemporal
boundary
formation
(SBF). The
authors report
4 experiments
exploring SBF.
The first 3
examine the
class of
transformation
s producing
SBF,
indicating
that local
element
changes in
color,
orientation,
or location
are all
effective. A
4th experiment
examines
temporal
constraints on
SBF.
Integration of
local element
changes to
produce
boundaries,
form, and
global motion
appears to be
confined to a
165-ms window.
Two classes of
spatiotemporal
integration
models are
considered;
the relation
between SBF
and other
cases of
boundary
interpolation
are discussed.
Source: Journal of experimental psychology. General, Vol. 123, No. 1. (March 1994), pp. 3-20. - Perceptions of
depth elicited
by occluded
and shearing
motions of
random dots: Perception,
Vol. 17
(1988), pp.
289-296.A
computer-contr
olled display
of random dots
was used to
study
perceptions of
depth. In this
display, a
field of
stationary
random dots
surrounded a
rectangular
area in which
random dots
moved with
uniform
velocity in a
single
direction. The
boundaries of
this rectangle
did not move.
When dot
motion was
perpendicular
to the longer
boundary of
the rectangle
(occluded
motion), the
rectangle
seemed to be
behind the
stationary
background
surround.
Motion
parallel to
the longer
boundary of
the rectangle
(shearing
motion) made
it appear in
front of the
surround. The
relative
lengths of the
sides of the
rectangle
determined
which effect
predominated.
Thus, for
motion
perpendicular
to the long
axis of the
rectangle the
occlusion
predominated
and naive
subjects
reported that
the central
area seemed
farther away
than the
surround. For
shearing
motion
parallel to
the long axis,
the subjects
reported that
the rectangle
was closer
than the
surround and
the strength
of both
effects also
depended on
the
length-to-widt
h ratio of the
rectangle. If
there was
occluded
motion along
the long axis,
as the
length-to-widt
h ratio
increased so
did the
likelihood
that subjects
would report
seeing the
rectangle
behind the
surround.
Conversely,
with shearing
motion along
the long axis,
increasing the
length-to-widt
h ratio
increased the
likelihood
that the
rectangle
would appear
unambiguously
in front of
the surround.
Some subjects
integrated the
two cues with
the resulting
perception
being a
rotating
cylinder. The
occlusion
effect was
stronger than
the shearing
effect. In
fact, this
'far' depth
effect was so
powerful that
it tended to
override
conflicting
depth cues
such as height
in the visual
field or
stereopsis.
The 'near'
depth effect
produced by
shearing
motion was
definite but
these other
depth cues
could often
override it.
Source: Perception, Vol. 17 (1988), pp. 289-296. - The impact of
accretion disk
winds on the
X-ray spectrum
of AGN: Part 2
- XSCORT +
Hydrodynamic
Simulations: (6 Oct
2008)abridged:
We use XSCORT,
together with
the
hydrodynamic
accretion disc
wind
simulation
from Proga &
Kallman
(2004), to
calculate the
impact that
the accretion
disk wind has
on the X-ray
spectrum from
a 1E8 solar
mass black
hole Active
Galactic
Nuclei (AGN)
accreting at
0.5 L/L_Edd.
The properties
of the
resulting
spectra depend
on viewing
angle and
clearly
reflect the
distinct
regions
apparent in
the original
hydrodynamic
simulation.
Very
equatorial
lines-of-sight
(l.o.s) are
dominated by
Compton
scattering and
nearly-neutral
absorption.
Polar l.o.s
result in
largely
featureless
spectra.
Finally, l.o.s
that intersect
the transition
region between
these extremes
have a wide
range of
absorption
features
imprinted on
the spectrum.
Both polar and
transition
region l.o.s
produce
spectra that
show
highly-ionized
,
blue-shifted,
Fe absorption
features that
are
qualitatively
similar to
features
observed in
the X-ray
spectra of a
growing number
of AGN. The
spectra
presented here
clearly
demonstrate
that current
simulations of
line driven
AGN accretion
disk winds
cannot
reproduce the
smooth soft
X-ray excess.
Furthermore,
they predict
that high
accretion rate
(L/L_Edd) AGN
are likely to
be strongly
affected by
obscuration,
in sharp
contrast to
the clean
picture that
is generally
assumed, based
on the
observed
relation
between the
opening angle
of the
molecular
torus and AGN
luminosity.
Source: (6 Oct 2008)
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