****Extended captions for ViewsBHs still images ****

		    www.astro.umd.edu/~rauch/ViewsBHs/stills

			  Kevin P. Rauch, JHU & UMD

			     rauch@astro.umd.edu


The following image captions provide broad, self-contained descriptions of
the images in the stills hierarchy. See the READMEs in this and the video
directory for more physics-oriented descriptions and additional information.
The images may be freely redistributed and used, for non-profit purposes,
subject to proper acknowledgement of the author.


All images in the stills directory tree display the appearance of a thin, flat
disk of hot, luminous gas being accreted by a black hole, as seen by an
observer outside the hole's event horizon (or ``point of no return'').
Four circular hot spots have been added to the otherwise smooth disk to aid in
visualizing the gravitational lensing distortions produced by the black hole.
The accretion disk is depicted using a false-color scale in which the color
correlates with the (apparent) temperature of the gas like the colors of the
rainbow: red is the coolest material, green represents intermediate
temperatures, and blue/white is the hottest. The scale is identical for all
images.  The following captions describe the specific viewing conditions for
each image, listed by its base name (e.g., `distort' refers to distort.jpg and
distort.png, etc.).



distort:
-------

    The view of an accretion disk surrounding a rapidly rotating black hole
(which spins with 99.8% of the maximum possible specific angular momentum for
such objects), as seen by a stationary observer well outside the horizon but
only slightly above the disk surface itself. The disk material rotates
counter-clockwise around the hole, and Doppler red- and blue-shifting of the
radiation emitted near the disk's inner edge is clearly visible. The warped,
apparently face-on section of the (flat!) disk is an optical illusion, the
result of light from the far side of the disk being bent into the observer's
line of sight by the black hole (an example of strong-field gravitational
lensing).


lensed:
------

    The view of an accretion disk surrounding a non-rotating (i.e.,
Schwarzschild) black hole, as seen by an observer hovering just above the
inner edge of the disk (at three times the horizon radius, the point at which
circular orbits become unstable for massive particles). The lower crescent is
in fact an image of the *underside* of the disk---the corresponding top surface
making up the above, main body of the picture. Note the extreme lensing
distortions (e.g., variations in ring width) of the bottom segment. A portion
of the next, much thinner ring image is also visible; formally, an infinite
sequence of images converging on the photon orbit (at 1.5 times the horizon
radius) is produced, whose successive widths decrease exponentially.


rings:
-----

    This image is similar to the preceding one (`lensed'), except that the
observer now views the disk face-on, from high above the disk, instead of 
near the disk plane at its inner edge (the black hole also rotates slowly,
at 25% the maximum value). Due to symmetry the ``ring'' images are now truly
uniform rings, and all four hot spots are visible in the first ring
image; an extremely thin second ring is also faintly visible. In the outer,
main portion of the disk, the (intrinsically circular!) hot spots appear
elliptical due to aberration and geometric time-delay effects. Note the lack
of visible Doppler effects due to the face-on viewing conditions.


subrings:
--------

    This image displays an extreme close-up of the first ring seen in the
previous picture (`rings'); the second ring is now clearly resolved, and 
just inside it the third ring image can be glimpsed (only marginally, in the
case of the VGA resolution images).


spiralFW:
--------

    The extraordinarily distorted and bewildering appearance of the same thin,
flat accretion disk displayed in the first image (`distort'), when viewed by
an observer co-rotating with the disk near the black hole (at 1.25 times the
horizon radius). As the gas in this region is very hot, blues dominate greens
and reds in the false-color image. The black hole looks more like a ``black
cylinder'' than a sphere, and multiple lensed images of the disk layer on top
of one another towards the center of the image. The ``hole'' at the left edge
of the frame (which coincides with the right edge in this cut-open cylindrical
plot) is actually the ``sky''---the lines of sight directed most nearly away
from the hole and towards infinity. Much of the extreme lensing distortions
result from the observer lying inside the photon orbit for most lines of sight
(since the radius of the photon orbit depends on its inclination for rotating
black holes, the ``location'' of the photon orbit depends on where precisely
you're looking).


spiralBK:
--------

    This image is identical to the previous one (`spiralFW'), except that the
image is now centered on the observer's back view (the ``sky,'' looking out
towards infinity) instead of on the black hole itself. Observing the universe
this close to a black hole, only a fine line separates the blackness of outer
space from the blackness of the hole's event horizon!