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This question is based on this description of the Cormoran Supermassive Black Hole in the Orion Arm's website:

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?

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    $\begingroup$ Are you sure that the website you are quoting isn't a work of fiction? $\endgroup$
    – D. Halsey
    Commented Sep 4, 2021 at 22:52
  • $\begingroup$ @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. $\endgroup$ Commented Sep 4, 2021 at 23:00
  • $\begingroup$ 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. $\endgroup$
    – Jim421616
    Commented Sep 5, 2021 at 23:43

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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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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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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, is in such a state.

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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