<p>As I understand the quote, the &quot;artificial&quot; thing about your accretion disk is not that it is bright, but that it doesn't emit X rays.</p>
<p>In real life, SMBH accretion disks are usually exceptionally bright. The disk itself has a strong emission in the UV, called UV bump, that can easily exceed the emission of the whole host galaxy.</p>
<p>Above and below the disk there is a region called corona, that emits powerful X radiation due to inverse Compton effect with the photons from the disk.</p>
<p>In the direction of the rotation axis, the Black hole may also emit relativistic jets that become huge lobes, way bigger than the hole galaxy and very bright in the radio as they sweep the gas of halo of the galaxy away.</p>
<p>Around the black hole, but farther that the accretion disk, there also are fast rotating gas clouds characterized by typical strong and high potential emission lines.</p>
<p>I hope I have given you the idea. Accreting SMBHs, also called Active Galactic Nuclei, are among the brightest objects in the universe across an impressive range of wavelengths.</p>

CC BY-SA 4.0
2021-09-04T17:56:45.143Z
2021-09-04T17:56:45.143Z
Prallax
<p>An accretion disc <em>would</em> be very bright!  In an accretion disc, matter is orbiting, and different parts of the disc move at different speeds. This causes friction, and at speeds that are a good fraction of the speed of light, the friction (driven by turbulence) in the accretion disc is <em>extreme</em>. It will heat the disc to millions of degrees. It will be extremely bright across the spectrum. In visible light it would be blue-white (though the light would be far to intense to look at). It would also give off large amounts of ultraviolet and X-radiation.</p>
<p>The inner parts of the disc are hottest, down to the last stable circular orbit.  At the velocities of the inner part of the disc, there will also be significant red and blue shifting if you view the disc from an angle, and gravitational lensing around the black hole.</p>
<p>(Sources  <a href="https://www.einstein-online.info/en/spotlight/accretion/" rel="nofollow noreferrer">https://www.einstein-online.info/en/spotlight/accretion/</a>   <a href="https://slideplayer.com/slide/1517144/" rel="nofollow noreferrer">https://slideplayer.com/slide/1517144/</a>)</p>

CC BY-SA 4.0
2021-09-04T17:54:06.910Z
2021-09-04T17:54:06.910Z
James K
<p>The accretion disk around a black hole can certainly be in a state where its x-ray emission is low and its optical emission is (relatively) high. Indeed, the accretion disk around the closest known black hole to Earth, <a href="https://en.wikipedia.org/wiki/A0620-00" rel="nofollow noreferrer">A0620-00</a>, is in <a href="https://arxiv.org/pdf/1711.04330.pdf" rel="nofollow noreferrer">such a state</a>.</p>
<p>This state is not stable: A0620-00 had an x-ray outburst in 1975, and probably also in 1917. Such a state for a supermassive black hole accretion disk would presumably also be unstable, but on a much longer timescale. We know of no such system, but it would be difficult to detect with present methods.</p>

CC BY-SA 4.0
2021-09-05T13:21:46.127Z
2021-09-05T13:21:46.127Z
John Doty
<p>This question is based on this description of the Cormoran Supermassive Black Hole in the <a href="https://www.orionsarm.com/eg-article/5bab765b00ec3?fbclid=IwAR22GF0OL_XqUdsGyepYpE-waApNIOLFrwr7SQBefN_qSbyy9rLCDTC4qLg" rel="noreferrer">Orion Arm's website</a>:</p>
<blockquote>
<p>Surrounding this event horizon is an (apparently artificial) accretion
disk about 4AU in diameter, which shines due to internal fusion, with
a surface temperature of 5700K and a luminosity of 4500 x Sol.  This
accretion disk is unusually deficient in x-ray emissions, and this
made detection relatively difficult; this deficiency cannot be
explained naturally, and was the first indication that this object was
subject to some form of advanced technological management.</p>
</blockquote>
<p>But based on what we know about black holes in real life, can the accretion disk be bright through natural means instead?</p>

CC BY-SA 4.0
2021-09-04T22:52:23.760Z
Are you sure that the website you are quoting isn't a work of fiction?
D. Halsey
CC BY-SA 4.0
2021-09-04T23:00:25.463Z
@D.Halsey  About the no x-rays?  That sounds like fiction.  As for the rest, I was just checking before I do my own worldbuilding.
JohnWDailey
CC BY-SA 4.0
2021-09-05T23:43:30.280Z
The Orion's Arm site is definitely fiction, albeit very interesting. Don't take anything it has to say on the matter of astrophysics too seriously.
Jim421616
<black-hole><supermassive-black-hole><luminosity><accretion-discs>
Could a black hole's accretion disk be bright through natural means?
JohnWDailey
# Could a black hole's accretion disk be bright through natural means?

(Asked by: username_0 on 2021-09-04)

This question is based on this description of the Cormoran Supermassive Black Hole in the [Orion Arm's website](https://www.orionsarm.com/eg-article/5bab765b00ec3?fbclid=IwAR22GF0OL_XqUdsGyepYpE-waApNIOLFrwr7SQBefN_qSbyy9rLCDTC4qLg):

> Surrounding this event horizon is an (apparently artificial) accretion
> disk about 4AU in diameter, which shines due to internal fusion, with
> a surface temperature of 5700K and a luminosity of 4500 x Sol. This
> accretion disk is unusually deficient in x-ray emissions, and this
> made detection relatively difficult; this deficiency cannot be
> explained naturally, and was the first indication that this object was
> subject to some form of advanced technological management.

But based on what we know about black holes in real life, can the accretion disk be bright through natural means instead?

**Tags:** black-hole, supermassive-black-hole, luminosity, accretion-discs
> **Comments:**
> **username_1**: Are you sure that the website you are quoting isn't a work of fiction?  
> **username_0**: @D.Halsey  About the no x-rays?  That sounds like fiction.  As for the rest, I was just checking before I do my own worldbuilding.  
> **username_2**: The Orion's Arm site is definitely fiction, albeit very interesting. Don't take anything it has to say on the matter of astrophysics too seriously.  

---

## 3 answers

### Answer by username_3 on 2021-09-04. Score: 6

As I understand the quote, the "artificial" thing about your accretion disk is not that it is bright, but that it doesn't emit X rays.

In real life, SMBH accretion disks are usually exceptionally bright. The disk itself has a strong emission in the UV, called UV bump, that can easily exceed the emission of the whole host galaxy.

Above and below the disk there is a region called corona, that emits powerful X radiation due to inverse Compton effect with the photons from the disk.

In the direction of the rotation axis, the Black hole may also emit relativistic jets that become huge lobes, way bigger than the hole galaxy and very bright in the radio as they sweep the gas of halo of the galaxy away.

Around the black hole, but farther that the accretion disk, there also are fast rotating gas clouds characterized by typical strong and high potential emission lines.

I hope I have given you the idea. Accreting SMBHs, also called Active Galactic Nuclei, are among the brightest objects in the universe across an impressive range of wavelengths.


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### Answer by username_4 on 2021-09-04. Score: 3

An accretion disc *would* be very bright! In an accretion disc, matter is orbiting, and different parts of the disc move at different speeds. This causes friction, and at speeds that are a good fraction of the speed of light, the friction (driven by turbulence) in the accretion disc is *extreme*. It will heat the disc to millions of degrees. It will be extremely bright across the spectrum. In visible light it would be blue-white (though the light would be far to intense to look at). It would also give off large amounts of ultraviolet and X-radiation.

The inner parts of the disc are hottest, down to the last stable circular orbit. At the velocities of the inner part of the disc, there will also be significant red and blue shifting if you view the disc from an angle, and gravitational lensing around the black hole.

(Sources <https://www.einstein-online.info/en/spotlight/accretion/> <https://slideplayer.com/slide/1517144/>)


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### Answer by username_5 on 2021-09-05. Score: 0

The accretion disk around a black hole can certainly be in a state where its x-ray emission is low and its optical emission is (relatively) high. Indeed, the accretion disk around the closest known black hole to Earth, [A0620-00](https://en.wikipedia.org/wiki/A0620-00), is in [such a state](https://arxiv.org/pdf/1711.04330.pdf).

This state is not stable: A0620-00 had an x-ray outburst in 1975, and probably also in 1917. Such a state for a supermassive black hole accretion disk would presumably also be unstable, but on a much longer timescale. We know of no such system, but it would be difficult to detect with present methods.


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