The picture from the Event Horizon Telescope:
https://apod.nasa.gov/apod/ap190411.html

Why is the center black?
Does the accretion disk just happen to be in a plane normal to our
line-of-sight?

(The near-perfect circularity of the image implies this
may be so, but the varying intensity implies not.)

In particular, if the accretion disk is not in a plane normal to our
line-of-sight, why don't we see light from the portion of it between us
and the black hole? And why isn't the image elliptical?

From the image, can they infer anything about the spin of the black
hole? How about from observing stars orbiting nearby?

Tom Roberts

[[Mod. note --
(Tom very likely knows all this, but others may not.)

1. VERY IMPORTANT: Because the observed photons (mm-wavelength radio
waves) originated close to the black hole, their paths were strongly
bent by the black hole's gravity. So, the appearance of the image
is very different than a geometric projection of the actual physical
positions from which the photons were emitted.

2. The observations basically measured the 2-D Fourier transform of
the sky's radio brightness, at a finite set of spatial frequencies
corresponding to the inter-telescope baselines projected on the sky
plane (see figure 2 of paper 1 in the list below, or paper 4 for many
more details). Reconstructing an image from this data is a tricky
inverse problem (see paper 4 for details).

3. Yes, this tells us a bit about the black hole spin and its orientation.
See paper 5 in the list below for details.

4. The first 5 research papers describing this are open-access at the
Astrophysical Journal Letters website: (There's mention of a paper 6
but I haven't found it yet)

First M87 Event Horizon Telescope Results.
I. The Shadow of the Supermassive Black Hole
https://iopscience.iop.org/article/1...41-8213/ab0ec7

First M87 Event Horizon Telescope Results.
II. Array and Instrumentation
https://iopscience.iop.org/article/1...41-8213/ab0c96

First M87 Event Horizon Telescope Results.
III. Data Processing and Calibration
https://iopscience.iop.org/article/1...41-8213/ab0c57

First M87 Event Horizon Telescope Results.
IV. Imaging the Central Supermassive
Black Hole
https://iopscience.iop.org/article/1...41-8213/ab0e85

First M87 Event Horizon Telescope Results.
V. Physical Origin of the Asymmetric Ring
https://iopscience.iop.org/article/1...41-8213/ab0f43

-- jt]]