On 12/04/2019 18:03, Tom Roberts wrote:
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.

Is there any prospect of computing a somewhat larger image zoomed out by
a factor of 3, 10 or 100 from the existing VLBI dataset?

The initial rough image on her facebook page has a tantalising point
source just north of the ring and about one diameter away.

https://www.facebook.com/photo.php?f...type=3&theater

The final published image was perhaps a bit close cropped.

A couple of things occur to me. M87 spin axis is pretty much pointed
towards us with the best estimate of 17 degrees off line of sight. So we
are in effect looking down into the throat of the jet engine.
(ignoring for the moment the huge GR ray tracing distortions)

Are there any EHT candidate radio galaxies near enough to image with the
spin axis perpendicular to our line of sight? Cygnus A is too far away.

Would the EHT be capable of taking a look at a starburst galaxy like M82
and making sense of the various odd compact objects lurking in there?
I'm guessing most of them would be in the beam of most of the antennae.

Or even closer to home could EHT do M1 the crab nebula and look into a
much smaller accretion disk very much closer to home. I guess temporal
variations in the emission might stymie any such attempt.

I presume SgrA* has caused problems because its emissions were varying
during the observations. Perhaps that limits the technique to a mere
handful of super massive black holes in relatively nearby galaxies.

It is an impressive achievement to image the accretion disk/black hole
shadow. It looks remarkably like the theoretical model predictions.

--
Regards,
Martin Brown

[[Mod. note -- Getting either higher resolution, or a wider field of
view (probably at lower resolution) would be somewhat difficult. The
problem is that (oversimplifying a bit), the observations measure the
2-D Fourier transform of the sky brightness, at spatial frequencies
given by the projections of each antenna-to-antenna baseline onto the
sky plane. These projections are time-dependent due to the Earth's
rotation.

So, given a small finite set of radio telescopes, and a finite time
span of observations, one gets measurements along only a finite set
of "tracks" in the spatial-frequency plane. These tracks are shown
in Figure 2 of the EHT collaboration's Paper I
( https://iopscience.iop.org/article/1...41-8213/ab0ec7 ).

Since interferometry is only possible with *simultaneous* observation
from different telescopes, it's restricted to times when the source
is simultaneously above the horizon for all the telescopes. So making
the individual spatial-frequency tracks longer by observing for longer
periods is probably impossible.

Thus, getting data at other spatial frequencies basically requires
finding (and getting time on) additional radio telescopes (with
suitable properties for these observations) in other parts of the
world, beyond those already used for these observations. That's
possible, but hard -- there aren't very many big millimeter-wave
radio telescopes in the world.
-- jt]]