Black hole at the centre of the galaxy imaged for the first time.

[Nicolaas Vroom]

In this article https://www.nature.com/articles/d41586-022-01320-y we can read:
"The long-awaited results, show an image etc. : a ring of radiation
surrounds a darker disk of precisely the size that was predicted from
indirect observations and from GR."
My question is: If you travel in a spaceship around this BH, like the
Sun travels around this BH, will you always observe more or less the
same ring? The same question in opposite direction?
My prediction is: Yes.

Nicolaas Vroom
https://www.nicvroom.be/

[Nicolaas Vroom]

Post by Nicolaas Vroom
My prediction is: Yes.

What that means that the BH is a spherical symmetrical object and that
there exists a 'gaseous' layer outside the radius of the BH which emits
light in all? directions. From the point of view of our earth we observe
this gaseous layer as a ring, but this view will be the same for every
observer, at the same distance as us, from the BH. In reality such a
physical ring, perpendicular to our line of sight, does not exist; its a
physical layer.

Nicolaas Vroom
https://www.nicvroom.be/

[Moderator's note: Almost all, or all, astrophysical black holes are not
spherically symmetric in the sense that they rotate. Rotating black
holes are more complicated. What an observer actually sees when looking
at a black hole is not trivial to calculate. In any case, the general
consensus is that black holes detectable via radiation emitted from near
them have an accretion disk and thus aren't spherically symmetric.
-P.H.]

[Nicolaas Vroom]

Post by Nicolaas Vroom
What that means that the BH is a spherical symmetrical object and that
there exists a 'gaseous' layer outside the radius of the BH which emits
light in all? directions.

SNIP

Post by Nicolaas Vroom
[Moderator's note: Almost all, or all, astrophysical black holes are not
spherically symmetric in the sense that they rotate.

My understanding is that also all stars and planets rotate, at the same
time many of these could be called spherical symmetric like our earth
and the Sun.

Post by Nicolaas Vroom
Rotating black holes are more complicated. What an observer actually
sees when looking at a black hole is not trivial to calculate.

The first step is to observe. To calculate is a second step.

Post by Nicolaas Vroom
In any case, the general
consensus is that black holes detectable via radiation emitted from near
them have an accretion disk and thus aren't spherically symmetric.

The question is to what extend can we conclude, based on observations,
that the BH, part of Sagittarius A*, has an accretion disk. Observing
the picture in https://www.nature.com/articles/d41586-022-01320-y the
ring surrounding the BH must be an 'indication' of this disk. If that is
the case the ring must be situated in a plane almost perpendicular
towards the direction of the line of sight between the earth and the
centre of the BH. (1) This direction must also be the same as the axis
of rotation of the BH.

Assuming that the ring is part of an accretion disk, I should expect,
that if we travel around this BH, like the Sun does around the BH, the
shape of this ring, as observed from our spaceship, must also change.
This shape must be almost the same after we have travelled 180 degrees
and the same after 360 degrees.

As I mentioned before, I have my doubts. The ring does not change and
there is no prove of an accretion disk, based on this image.

What also is in favour of a sperical object is that the movement of the
stars around the BH is random. There is no preference.

I found also a different article:
https://www.nasa.gov/feature/goddard/2021/hubble-mini-jet-found-near-milky-ways-supermassive-black-hole
This article also shows the direction of rotation of the BH.
The direction is different compared with (1) above.

My impression is that when you read this article and other articles the
accretion disks are of temporary nature and depend about source, that
causes the inflow of material. Together the BH and the source can be
considered as a binary system.

As mentioned above the movement of the stars around Sagitarrius A* are
random, as such, my guess is, that the direction of possible accretion
disks is also random, which is in contradiction with observation (1)

Nicolaas Vroom
https://www.nicvroom.be

[[Mod. note -- A few comments:
1. The Earth is *approximately* spherically symmetric, but if you look
more closely it's shape is in fact rotationally flattened. That is,
the Earth's equatorial radius is about 0.34% larger than its polar
radius, so the Earth is in fact NOT spherically symmetric.

2. While it's true that if we travel around the Sgr A* BH, its apparent
shape will change, that doesn't help us right now: our solar system
takes around 250 million years to orbit the center of our galaxy,
so we're not going to get to look at the Sgr A* BH from a
significantly different orientation any time in our lives.

3. Accretion disks (including the one around the Sgr A* BH) are indeed
temporary and depend on the availablity of source matter. But I
wouldn't say that the BH and the source are a "binary" system, because
there's no reason to think that the source is a single compact object.
Rather, the BH is embedded in a cloud of (moving) stars and
interstellar gas.

4. This 2020 article by Fragione & Loeb,
https://iopscience.iop.org/article/10.3847/2041-8213/abb9b4
(which argues for a relatively low (slow) spin for the Sgr A* BH)
notes that past studies have given conflicting values for that spin.
I don't know enough about this subject to have an informed opionion
myself. Given the instruments now operational, we should know a
*lot* more about this in a few years, especially once ESO's
Extremely Large Telescope is operational (planned for 2027ish).
-- jt]]

[Martin Brown]

Post by Nicolaas Vroom

Post by Nicolaas Vroom
What that means that the BH is a spherical symmetrical object and that
there exists a 'gaseous' layer outside the radius of the BH which emits
light in all? directions.

SNIP

Post by Nicolaas Vroom
[Moderator's note: Almost all, or all, astrophysical black holes are not
spherically symmetric in the sense that they rotate.

My understanding is that also all stars and planets rotate, at the same
time many of these could be called spherical symmetric like our earth
and the Sun.

Apart from having a spin axis... They are oblate spheroids for the most
part and become more so the faster that they are spinning.

In our own solar system Saturn and Uranus both have equatorial rings -
the former being its most spectacular feature.

Post by Nicolaas Vroom

Post by Nicolaas Vroom
Rotating black holes are more complicated. What an observer actually
sees when looking at a black hole is not trivial to calculate.

The first step is to observe. To calculate is a second step.

We can observe directly a close analogue of a spinning black hole in the
Crab nebula pulsar and with enough resolution in X-rays to see both the
accretion disk and jets coming from the poles. Neutron stars are only a
relatively modest factor of about 3 short of being black holes. Drop
enough extra mass onto them from a nearby star and they may become one.

https://chandra.harvard.edu/photo/2017/crab/

Select X-ray

Chandra has also imaged Sgr A* and that puts bounds on how much matter
in its vicinity actually goes down the plug hole (~1% at most).

https://chandra.harvard.edu/photo/2013/sgra_gas/

This URL may help answer most of the OPs questions:

https://www.space.com/sagittarius-a

Post by Nicolaas Vroom

Post by Nicolaas Vroom
In any case, the general
consensus is that black holes detectable via radiation emitted from near
them have an accretion disk and thus aren't spherically symmetric.

The question is to what extend can we conclude, based on observations,
that the BH, part of Sagittarius A*, has an accretion disk. Observing
the picture in https://www.nature.com/articles/d41586-022-01320-y the
ring surrounding the BH must be an 'indication' of this disk. If that is
the case the ring must be situated in a plane almost perpendicular
towards the direction of the line of sight between the earth and the
centre of the BH. (1) This direction must also be the same as the axis
of rotation of the BH.

It would be incredibly surprising if it did not have an accretion disk
of some sort if there is any matter near enough to be subject to being
pulled in. It has to lose angular momentum somehow to fall into it.

The black hole has strong enough gravity to bend light paths over the
poles so that you see something that is quite distorted from whatever
angle you look. Raytracers have simulated this. I am surprised how close
to a blurred version of their predictions the observations have been!

Sera Markoff's page has a nice movie of how the appearance of the (M87
BH) would change with observing wavelength.

https://www.seramarkoff.com/2022/02/exploring-the-appearance-of-black-hole-by-ray-tracing/

I am more concerned with the dynamical timescales making the intrinsic
assumptions of aperture synthesis invalid for Sgr A*. I know they imaged
it in snapshot mode to try and avoid these issues. ISTR the images
obtained clustered around certain specific patterns of brightness.
A few are shown in Fig 3 here. I'm sure there is a larger set somewhere.

https://iopscience.iop.org/article/10.3847/2041-8213/ac6674/pdf

Post by Nicolaas Vroom
Assuming that the ring is part of an accretion disk, I should expect,
that if we travel around this BH, like the Sun does around the BH, the
shape of this ring, as observed from our spaceship, must also change.
This shape must be almost the same after we have travelled 180 degrees
and the same after 360 degrees.

Not if the thing is interacting with matter. Bright spots on the
accretion disk may change on timescales worryingly close to the time
required to obtain enough data for a satisfactory image of the target.

By comparison the core of M87 is about a thousand times bigger and also
a thousand times further away so although about the same apparent size
on the sky viewed from Earth is much more stable in its appearance.

Post by Nicolaas Vroom
As I mentioned before, I have my doubts. The ring does not change and
there is no prove of an accretion disk, based on this image.
What also is in favour of a sperical object is that the movement of the
stars around the BH is random. There is no preference.

Far enough away from the BH it is almost indistinguishable from a point
mass as far as its gravitational dynamics are concerned. A small amount
of frame dragging could be detectable but that becomes a much more
significant effect when they are closest. Has any dynamical evidence of
frame dragging been seen on any stars making very close approaches?
(my guess is we don't have the resolution to be able to tell)

It would be fun to see what happens if a star does get too close and is
shredded and the whole thing lights up brightly for a while.

Post by Nicolaas Vroom
https://www.nasa.gov/feature/goddard/2021/hubble-mini-jet-found-near-milky-ways-supermassive-black-hole
This article also shows the direction of rotation of the BH.
The direction is different compared with (1) above.
My impression is that when you read this article and other articles the
accretion disks are of temporary nature and depend about source, that
causes the inflow of material. Together the BH and the source can be
considered as a binary system.
As mentioned above the movement of the stars around Sagitarrius A* are
random, as such, my guess is, that the direction of possible accretion
disks is also random, which is in contradiction with observation (1)

Even if matter is injected into the accretion zone at a very oblique
angle it will fairly quickly be spread out along its orbit and then
settle down into an equatorial ring or donut due to friction. The spin
of a black hole causes strong frame dragging in close proximity to it.

Also quite likely to have a ferocious magnetic field as well.

[Nicolaas Vroom]

Post by Martin Brown

Post by Nicolaas Vroom
The question is to what extend can we conclude, based on observations,
that the BH, part of Sagittarius A*, has an accretion disk. Observing
the picture in https://www.nature.com/articles/d41586-022-01320-y the
ring surrounding the BH must be an 'indication' of this disk. If that is
the case the ring must be situated in a plane almost perpendicular
towards the direction of the line of sight between the earth and the
centre of the BH. (1) This direction must also be the same as the axis
of rotation of the BH.

It would be incredibly surprising if it did not have an accretion disk
of some sort if there is any matter near enough to be subject to being
pulled in. It has to lose angular momentum somehow to fall into it.

But during that process light can be emitted in all directions and
the light we see is more or less emitted in our direction.

[[Mod. note -- You're mistaken. The light we is is that which
*eventually* is pointing in our direction, but it may have been
emitted in a very different direction (and then had its path bent by
the strong gravitational field into one pointing in our direction).
-- jt]]

Post by Martin Brown
The black hole has strong enough gravity to bend light paths over the
poles so that you see something that is quite distorted from whatever
angle you look. Raytracers have simulated this. I am surprised how close
to a blurred version of their predictions the observations have been!

That is correct.
But this light can come from all directions and also be emitted in all
directions. Part of that emitted light can come in our direction.

Post by Martin Brown

Post by Nicolaas Vroom
Assuming that the ring is part of an accretion disk, I should expect,
that if we travel around this BH, like the Sun does around the BH, the
shape of this ring, as observed from our spaceship, must also change.
This shape must be almost the same after we have travelled 180 degrees
and the same after 360 degrees.

Not if the thing is interacting with matter. Bright spots on the
accretion disk may change on timescales worryingly close to the time
required to obtain enough data for a satisfactory image of the target.

My assumption is that this accretion disc is more or less fixed to the
BH and lies in the plane of the picture. That means if you travel in 80 days
around this BH that when you return after 80 days the picture should
be more or less the same. But the intermediate pictures should not.
If you start from a circle after 10 days this should be an ellipse after
20 days a vertical beam, after 30 days an ellipse and after 40 days again
a circle. What I mean is that to observe a circle is rare.

At the same time, that is my guess, if the BH would be surrounded, with
a more or less equally distributed layer of some gaseous material, it
is possible that you always observe this more or less doughnut shaped
visible ring.

Regards
Nicolaas Vroom.

[Eric Flesch]

On Sat, 09 Jul 2022 22:16:32 PDT, Nicolaas Vroom

Post by Nicolaas Vroom
[[Mod. note -- You're mistaken. The light we is is that which
*eventually* is pointing in our direction, but it may have been
emitted in a very different direction (and then had its path bent by
the strong gravitational field into one pointing in our direction).
-- jt]]

Is this not a philosophic point? What is more "our direction" than
the null geodesic which connects the light source to us? How do we
define the "straightness" which is *more* in "our direction"?

Mach's Principle gets in here, where it holds that there can be no
space wihout some matter to occupy it, i.e., matter is an inherent
part of any complete spatial manifold. Given that, it's pretty hard
to define something "straighter" than the null geodesic contoured by
the essential matter.


[[Mod. note --
1. That's not really what Mach's principle says. Among other things,
a "complete" spatial manifold (which implies that it doesn't contain
any black holes) may be a vacuum (contain no matter), but still
contain spacetime curvature (nonzero Riemann tensor), e.g.,
gravitational waves and/or geons
https://en.wikipedia.org/wiki/Geon_(physics)

2. There are useful notions of "direction" other than those of null
geodesics. For example,
(a) Kerr spacetime is reflection-symmetric about the equator, and
hence "the equator" is a *physically* defined place (set of events)
in Kerr spacetime (i.e., it's one which can be defined uniquely
regardless of the coordinate system in use). And,
(b) Far from the black hole (the asymptotically-flat region) we
have a well-defined sret of nearly-Minkowskian (flat-spacetime)
coordinates, so it's meaningful to talk about things like the
z coordinate (with respect to the equator of a BH whose spin axis
is vertical) of a light ray which is moving horizontally. We
often call this the light ray's "impact parameter".

Putting these together, we can have a situation like this (forgive
the crude ASCII-art; this is best viewed with a monopitch font)):


-----------------------------------
---------------
------
/
// *****
/ *********
+ *********
*********
*****


Here I've shown a side view of a Kerr black hole (denoted by asterisks),
i.e., the BH's spin axis is vertical. A null geodesic originates on the
equator (z=0) at the left and curves over the BH's north pole, arriving
at r=infinity on the right moving horizontally with some impact parameter
b > 0.

Since this light originates on the equator, and winds up in the
asymptotically-flat region travelling parallel the equator but offset
to a nonzero impact parameter, I it's reasonable to say that the
light's path has been bent by the gravitational field.
-- jt]]

[Phillip Helbig (undress to reply)]

Post by Eric Flesch

Post by Nicolaas Vroom
[[Mod. note -- You're mistaken. The light we is is that which
*eventually* is pointing in our direction, but it may have been
emitted in a very different direction (and then had its path bent by
the strong gravitational field into one pointing in our direction).
-- jt]]

Is this not a philosophic point? What is more "our direction" than
the null geodesic which connects the light source to us? How do we
define the "straightness" which is *more* in "our direction"?
Mach's Principle gets in here, where it holds that there can be no
space wihout some matter to occupy it, i.e., matter is an inherent
part of any complete spatial manifold. Given that, it's pretty hard
to define something "straighter" than the null geodesic contoured by
the essential matter.
[[Mod. note --
1. That's not really what Mach's principle says. Among other things,
a "complete" spatial manifold (which implies that it doesn't contain
any black holes) may be a vacuum (contain no matter), but still
contain spacetime curvature (nonzero Riemann tensor), e.g.,
gravitational waves and/or geons
https://en.wikipedia.org/wiki/Geon_(physics)

Another example are the Friedmann cosmological models (homogeneous and
isotropic models based on General Relativity). They can be empty
(contain no matter) but still have (spatial or spacetime) curvature.

[Nicolaas Vroom]

SNIP

Post by Nicolaas Vroom
1. The Earth is *approximately* spherically symmetric, but if you look
more closely its shape is in fact rotationally flattened. That is,
the Earth's equatorial radius is about 0.34% larger than its polar
radius, so the Earth is in fact NOT spherically symmetric.

I expect the 'same' can be said of the Sgr A* BH, but very difficult
to prove, based on observational evidence.

Post by Nicolaas Vroom
2. While it's true that if we travel around the Sgr A* BH, its apparent
shape will change, that doesn't help us right now: our solar system
takes around 250 million years to orbit the centre of our galaxy,
so we're not going to get to look at the Sgr A* BH from a
significantly different orientation any time in our lives.

My idea is to travel more in 80 days around the Sgr A* BH and make each day
a picture as shown in https://www.nature.com/articles/d41586-022-01320-y.
My expectation that the 80 pictures will be almost the same.
A different way to travel around the BH is in the same plane as the picture,
as observed ring, at the same distance as we are at the present.
My expectation that these 80 pictures also will be almost the same.
Two options: A ring or a vertical thick line. I expect a ring.
That means the ring around the dark circle in the centre is not a physical
ring but an image of the light, originating from the surroundings around
the BH, travelling in our directions.
The most probably explanation, if the pictures are almost the same,
that the surroundings of the BH are 'spherical' the same.
The consequence is that this is not an image of a BH. But this is open
for discussion. (That does not mean there is no BH)
If it was a physical ring the pictures should not be the same.

A picture of the BH M87 also shows a ring.

Post by Nicolaas Vroom
3. Accretion disks (including the one around the Sgr A* BH) are indeed
temporary and depend on the availability of source matter. But I
wouldn't say that the BH and the source are a "binary" system, because
there's no reason to think that the source is a single compact object.
Rather, the BH is embedded in a cloud of (moving) stars and
interstellar gas.

I agree with you. My main reason, why I'm interested in s-stars, starts
if you compare S62 with for example S6. The results of
my simulations show that the gravitational field of S6 influences
the behaviour of S62. S62 is a star which revolves in about 10 years
around Sgr A* BH while S6 does this in about 192 years. This variable
gravitational field is visible in the form of a gravitational wave.
Select this link:
https://www.nicvroom.be/VB2019%20Sagittarius.program.htm#par%206.6

Post by Nicolaas Vroom
4. This 2020 article by Fragione & Loeb,
https://iopscience.iop.org/article/10.3847/2041-8213/abb9b4
(which argues for a relatively low (slow) spin for the Sgr A* BH)
notes that past studies have given conflicting values for that spin.
I don't know enough about this subject to have an informed opinion
myself. Given the instruments now operational, we should know a
*lot* more about this in a few years, especially once ESO's
Extremely Large Telescope is operational (planned for 2027ish).
-- jt]]

A good article to read is regarding this subject is:
"X-ray astronomy comes of age"
https://www.nature.com/articles/s41586-022-04481-y
Here we can read at page 265:
Sgr A*, the SuperMassiveBH in the centre of the Milky Way is currently
in a radiatively inefficient accretion phase.... Less than 1% of this
gas accretes onto the SMBH, the remainder being ejected in a polar outflow...
The number of bright flares seen by Chandra and XMM-Newton increased about
six months after the closest approach of the gas cloud G2 to Sgr A*, which
suggests that its passage triggered additional accretion.

Nicolaas Vroom
http://www.nicvroom.be/

[Martin Brown]

Post by Nicolaas Vroom

Post by Martin Brown
The black hole has strong enough gravity to bend light paths over the
poles so that you see something that is quite distorted from whatever
angle you look. Raytracers have simulated this. I am surprised how close
to a blurred version of their predictions the observations have been!

That is correct.
But this light can come from all directions and also be emitted in all
directions. Part of that emitted light can come in our direction.

Some (perhaps even quite a lot of) relativistic beaming is likely from
the hotspots nearest the event horizon. Side of the disk coming towards
us will tend to appear both brighter and blue shifted AOTBE.

[[Mod. note -- I've never seen the anacronym "AOTBE" before, but
$SEARCH_ENGINE informs me it typically means "all other things being equal".
-- jt]]

Post by Nicolaas Vroom

Post by Martin Brown

Post by Nicolaas Vroom
Assuming that the ring is part of an accretion disk, I should expect,
that if we travel around this BH, like the Sun does around the BH, the
shape of this ring, as observed from our spaceship, must also change.
This shape must be almost the same after we have travelled 180 degrees
and the same after 360 degrees.

Not if the thing is interacting with matter. Bright spots on the
accretion disk may change on timescales worryingly close to the time
required to obtain enough data for a satisfactory image of the target.

My assumption is that this accretion disc is more or less fixed to the
BH and lies in the plane of the picture. That means if you travel in 80 days

That sort of rigid structure could not survive at all in close proximity
to a black hole (or for that matter any other gravitating body).

Every particle is in orbit in its own right. The accretion disk is
suffering insane sheer forces and turbulence in all probability.

[[Mod. note -- I think you meant "shear" forces. -- jt]]

The accretion disk is almost certainly spinning in the same sense as the
black hole and as such the last stable circular orbit is just above the
event horizon and travelling at the speed of light. I'd hazard a guess
that the inner section of the accretion disk is pretty much plasma in
almost circular orbits slowly spiralling in towards oblivion.

Post by Nicolaas Vroom
around this BH that when you return after 80 days the picture should
be more or less the same. But the intermediate pictures should not.
If you start from a circle after 10 days this should be an ellipse after
20 days a vertical beam, after 30 days an ellipse and after 40 days again
a circle. What I mean is that to observe a circle is rare.

There is no rigid disk. All particles are in orbit. This seems like a
decent observational paper of Sgr A* working at the limits of the VLT:

https://www.aanda.org/articles/aa/full_html/2018/10/aa34294-18/aa34294-18.html#FN1

Post by Nicolaas Vroom
At the same time, that is my guess, if the BH would be surrounded, with
a more or less equally distributed layer of some gaseous material, it
is possible that you always observe this more or less doughnut shaped
visible ring.

Although such a structure would give you the appearance of a ring for
example like M57 the Ring nebula or various supernova remnants it would
not fit with the physics of this situation in close proximity to a
spinning supermassive black hole.

The environment around a Kerr metric BH is anything but isotropic. It
would be great fun to see it swallow a star or gas cloud and illuminate
itself more fully. I wonder how long we will have to wait for that?