Black holes

[Granada]

I've started watching a great doco on Netflix called Black holes - The edge of all we know which is a story of how scientists captured the first image of a black hole.


One statement they made in passing caught my attention. They said in order to capture this image they utilised a number of large and powerful telescopes across the planet and pointed them all at the same time and location to "collect photons". I'm wondering since black holes don't emit anything but Hawking radiation, which photons are they referring to? Could it be the photons emitted by the stuff around the black hole, which is what we ended up seeing in the image they captured?

[Lariliss (Larissa)]

Exactly a united effort makes history.
Inside the black hole image that made history by Sheperd Doeleman and an international team.
This evidence makes a positive view of the future. Since there is a lot of data collection made, to achieve breakthrough collaboration is essential.
An international team anchored by the Event Horizon Telescope (EHT) Collaboration, which is known for capturing the first image of a black hole in the galaxy Messier 87, has now imaged the heart of the nearby radio galaxy Centaurus A in unprecedented detail.
To observe the Centaurus A galaxy with this unprecedentedly sharp resolution at a wavelength of 1.3 mm, the EHT collaboration used Very Long Baseline Interferometry (VLBI), the same technique with which the famous image of the black hole in M87 was made. An alliance of eight telescopes around the world joined together to create the virtual Earth-sized Event Horizon Telescope. The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, North and South America.

[Lariliss (Larissa)]

Originally Posted by Granada

which photons are they referring to? Could it be the photons emitted by the stuff around the black hole, which is what we ended up seeing in the image they captured?

The captured photons are of the matter around the black hole. Astronomers have also measured polarization close to the edge of a black hole (event horizon). This is a breakthrough in the observations and measurements of the black holes and more results to come from the obtained data analysis.

[Lariliss (Larissa)]

The captured photons are of the matter around the black hole. Astronomers have also measured polarization close to the edge of a black hole (event horizon). This is a breakthrough in the observations and measurements of the black holes and more results to come from the obtained data analysis.

[ChrisDecrow (Christopher)]

I suppose, they mean the photon sphere around black hole. This is a spherical boundary of zero thickness in which photons that move on tangents to that sphere would be trapped in a circular orbit about the black hole. For non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. While light can still escape from the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by the black hole. Hence any light that reaches an outside observer from the photon sphere must have been emitted by objects between the photon sphere and the event horizon.

[Lariliss (Larissa)]

Most of the black holes are believed to be rotating, since they are formed from collapsing stars, and all stars known to be rotating.

Rotating black hole is measured by mass and spin (0 to mod [1 - max])).

Additional matter, falling into a black hole also contributes to its rotation.
The mass of a black hole is estimated by its influence on other far bodies. Spin is harder to measure than mass, as it can be measured only with the closest particles to the black hole.
Innermost Stable Circular Orbit (ISCO), closer than this no orbits are stable, and falling into a black hole. The faster it spins the smaller the ISCO becomes. The spin is supporting the particles against the gravity.

The greater the spin, the closer the ISCO shrinks, up to the event horizon. ISCO’s less than event horizon haven’t been seen yet.

So, ISCO and black hole spin are related. One of the closest to the maximum black hole spin detected is 0.98 (GRS 1915+105).
The radii of ISCO discs are measured in the same way as the stars are measured.

[Shiraz (Ray)]

From
https://www.jpl.nasa.gov/edu/news/20...-a-black-hole/

"In the popular imagination, it was thought that capturing an image of a black hole was impossible because an image of something from which no light can escape would appear completely black. For scientists, the challenge was how, from thousands or even millions of light-years away, to capture an image of the hot, glowing gas falling into a black hole."

[xelasnave]

They...who ever "they" are have been capturing the hot glowing gas falling into a black hole for many years now...think quasars...
Alex

[Shiraz (Ray)]

Originally Posted by xelasnave

They...who ever "they" are have been capturing the hot glowing gas falling into a black hole for many years now...think quasars...
Alex

Agreed, but the difference here is that "they" imaged not a vast early and fiery accretion disk (as in a quasar), but the actual black hole silhouette at the centre of a relatively tame accretion region.

The angular resolution was about 20 micro arcseconds - to get that fine a resolution and still have enough sensitivity to see anything was amazing. "One microarcsecond is about the size of a period at the end of a sentence if you were looking at it from Earth and that period was in a leaflet left on the moon"
from https://www.livescience.com/65211-qu...f%20New%20York.)

Cheers Ray

[xelasnave]

Yes Ray absolutely amazing stuff...but the idea re quasar just "popped" into my head and I found it funny is all.
I should have been clearer I guess.
Alex

[Shiraz (Ray)]

No, you are right. In answer to Granada, light from the black hole regions
in Seyfert galaxies and quasars apparently have the same source of photons (hot accretion region material). The big difference is the extreme resolution of the black hole imaging effort.

Cheers Ray

[Lariliss (Larissa)]

Actually, many detected black holes are so called ‘black hole candidates’, which need further precise observation with high resolution.
I think, this explanation is fair enough:

https://esahubble.org/science/black_holes/

‘Today most astronomers believe that quasars, radio galaxies and the centres of so-called active galaxies just are different views of more or less the same phenomenon: a black hole with energetic jets beaming out from two sides. When the beam is directed towards us we see the bright lighthouse of a quasar. When the orientation of the system is different we observe it as an active galaxy or a radio galaxy. This ‘unified model’ has gained considerable support through a number of Hubble observational programs. The simplistic early ideas have however been replaced by a more complex view of this phenomenon – a view that will continue to evolve in the years to come.’