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EHT Picture of the black hole in M87



 
 
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  #1  
Old April 12th 19, 06:03 PM posted to sci.astro.research
Tom Roberts
external usenet poster
 
Posts: 344
Default EHT Picture of the black hole in M87

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]]
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  #2  
Old April 13th 19, 12:41 PM posted to sci.astro.research
Jonathan Thornburg [remove -animal to reply][_3_]
external usenet poster
 
Posts: 136
Default EHT Picture of the black hole in M87

In a recent moderator's note, I wrote
[[about the Event Horizon Telescope announcement of the first
"picture of a black hole"]]
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


paper 6 (also Astrophysical Journal Letters open-access) is at
First M87 Event Horizon Telescope Results.
VI. The Shadow and Mass of the Central Black Hole
https://iopscience.iop.org/article/1...41-8213/ab0f43

and there's a very informative commentary/synopsis of all the results at
Focus on the First Event Horizon Telescope Results
https://iopscience.iop.org/journal/2...e/Focus_on_EHT

ciao,

--
-- "Jonathan Thornburg [remove -animal to reply]"
Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA
"He wakes me up every morning meowing to death because he wants to
go out, and then when I open the door he stays put, undecided, and
then glares at me when I put him out"
-- Nathalie Loiseau (French minister for European Affairs,
explaining why she named her cat "Brexit")

  #3  
Old April 15th 19, 02:57 AM posted to sci.astro.research
Hans Aberg[_3_]
external usenet poster
 
Posts: 18
Default EHT Picture of the black hole in M87

On 2019-04-12 19:03, 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?


There is a good explanation of the expected image at [1], incidentally
published before the black hole image. In short, relativistic effects
cause the image.

1. https://www.youtube.com/watch?v=zUyH3XhpLTo
  #4  
Old April 19th 19, 10:01 PM posted to sci.astro.research
Martin Brown[_3_]
external usenet poster
 
Posts: 189
Default EHT Picture of the black hole in M87

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]]
  #5  
Old April 25th 19, 06:18 PM posted to sci.astro.research
Martin Brown[_3_]
external usenet poster
 
Posts: 189
Default EHT Picture of the black hole in M87

On 19/04/2019 22:01, Martin Brown wrote:

[snip]

I know it is bad form to reply to one's own post but here goes.

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.

][[Mod. note -- Getting either higher resolution, or a wider field of
]view (probably at lower resolution) would be somewhat difficult. The

Getting any higher resolution would be impossible. They have already
pushed the data just about as far as it will go in that direction.

I don't see why they can't map a slightly wider region though. It will
obviously look rather scrappy due to the sparse VLBI u-v coverage. The
zone around the phase centre should be OK for a few mas or so.

]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.

I should perhaps declare an interest in that long ago I wrote software
for aperture synthesis and I have followed M87 jet VLBI for a while.

]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 ).

Looking at that u-v coverage it strikes me that a larger image with some
horrendous hexagonal artefacts ought to be possible. It may be that the
emissions at 1.3mm are just too faint other than in the accretion disk.

Here is the earlier 3mm 100GHz VLBI series of results published in 2016
for a somewhat wider field of view:

https://arxiv.org/pdf/1609.07896.pdf

I was hoping that the new 1.3mm dataset would have been just about
sufficient to image a region approximately one third of that size IOW
the BH and the first hotspot/plateau at the very start of the jets.

Also similar results at 86GHz at 3.5mm:

https://iopscience.iop.org/article/1.../817/2/131/pdf

There is also the VLBI movie at twice that wavelength 7mm 43GHz:

http://www.aoc.nrao.edu/~cwalker/M87...vies_only.html

I guess something suddenly gets much tougher at the shortest wavelength
- in some ways it is astonishing that they can make it work at all.

]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]]

Agreed. But I am a bit puzzled what the practical differences are
between ETH operations at 1.3mm and the earlier 3mm VLBI work.

The ETH processing has concentrated on absolute maximum resolution of
fine detail in the highest signal to noise region to get that amazing
image of the accretion disk/shadow. But once they have a basic phase
solution why can't they make a crude image of a slightly wider region?

I'm surprised that there hasn't been any further discussion of the the
M87 results here beyond your own moderator's notes (for which thanks)...

--
Regards,
Martin Brown
  #6  
Old April 25th 19, 11:05 PM posted to sci.astro.research
Steve Willner
external usenet poster
 
Posts: 1,172
Default EHT Picture of the black hole in M87

In article ,
Martin Brown writes:
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.


M87 and Sgr A* were chosen because they are by far the best
candidates. I don't know what the next best would be.

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?


The southern hemisphere telescopes -- ALMA being by far the most
important -- can't look at M82. Even if they could, I doubt there
would be any sources with the enormous brightness temperature
required to give a signal.

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.


That and again perhaps brightness temperature.

I presume SgrA* has caused problems because its emissions were varying
during the observations.


That's my guess too, but the EHT collaboration hasn't said anything
so far as I know. We know from the GRAVITY results
https://ui.adsabs.harvard.edu/abs/20.....10G/abstract
that emission in the Sgr A* accretion disk varies on 10-minute time
scales.

It is an impressive achievement to image the accretion disk/black hole
shadow.


Indeed. They are in effect synchronizing telescopes a whole earth
apart to a fraction of a millimeter of light travel time. The
hydrogen maser clocks used for that are only one of the amazing
technical achievements that were needed to make EHT work.

[[Mod. note -- Getting either higher resolution, or a wider field of
view (probably at lower resolution) would be somewhat difficult.


I wasn't sure what limits the field of view, so I ask an EHT
expert. The response was:
For VLBI the FoV is often set by bandwidth and integration
time. This is because widely separated structure on the sky causes
high frequency corrugations on the fourier plane, and these will be
averaged over if the spanned bandwidth or averaging time is too
long. But these limits are typically fairly large - much larger
than the FoV we used. We did search for larger scale structure and
didn't find any.

That last doesn't surprise me. As mentioned above, the brightness
temperature has to be huge for VLBI to see anything.

UV coverage doesn't seem to be a limitation: at least little more for
imaging far from the phase center than near it. Bandwidth smearing
and time-average smearing are factors in conventional
interferometry. They can be overcome with a combination of more
complex equipment and higher data rates, but EHT is already pushing
data rate hard. I think the real limit here is that there just
aren't high T_b sources far from the center.

Higher resolution requires either shorter wavelengths -- challenging
but perhaps 0.8 or 0.9 mm might be possible -- or larger baselines.
That could in principle be done from space, but it wouldn't be quick
or cheap.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #7  
Old April 27th 19, 08:55 AM posted to sci.astro.research
Hannu Poropudas[_2_]
external usenet poster
 
Posts: 26
Default EHT Picture of the black hole in M87

On Monday, April 15, 2019 at 4:57:15 AM UTC+3, Hans Aberg wrote:
On 2019-04-12 19:03, 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?


There is a good explanation of the expected image at [1], incidentally
published before the black hole image. In short, relativistic effects
cause the image.

1. https://www.youtube.com/watch?v=zUyH3XhpLTo


I notice possible one reference about above link ?

Luminet Jean-Pierre, 1999.
Black Holes.
Cambridge University Press, (1992), reprint 1999.
pages 137-146 (10. Illuminations).

Best Regards,

Hannu Poropudas

 




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