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What is the light source in the black hole images?

I've searched the site before asking, as well as tried to google it. But perhaps the answer is buried due to the recent news.

My understanding, is that the ring in the images is an illuminated accretion disc.

What I could not find explained is the light source, and by extension, why is a bright accretion disc – which is round by definition – a proof for light bending? (I'm not disputing the fact.)

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  • $\begingroup$ I don't know if this helps, but from what I read, the people who worked on this decided themselves what color it would be in the picture. $\endgroup$
    – Brōtsyorfuzthrāx
    Apr 11, 2019 at 21:48

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The radiation being detected in this case is mostly synchroton radiation, caused by energetic electrons spiralling around magnetic field line, rather than thermal radiation, but it comes from the same place (the disk). The actual evidence for light bending is, I believe the dark area in the middle, which is several times larger than the actual event horizon and is caused because the light that would have been coming to us from that direction has been bent away.

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Accretion disks are a complicated system which produces a wide variety of emission across the electromagnetic spectrum on a variety of timescales. The diagram below shows (some) of what is going on schematically, along with typical sizes scales (bars with arrows with sizes in parsecs (pc)). Details about how the "central engine" of supermassive black holes and AGN works and exactly how the energy is generated is an ongoing topic of research, which the Event Horizon Telescope images will very likely contribute to.

Schematic diagram of an accretion disc Friction and turbulence from the material orbiting the black hole causes the accretion disk to heat up and emit light. The inner parts of the accretion disk get the hottest and tend to emit at the shortest wavelengths (X-rays normally) with longer wavelength (optical, IR, radio) normally coming from the material further out and above the disk which is photo-ionized by the X-rays. Depending on the density of the material in and around the accretion disk, the angle we are viewing it from and whether the accretion disk flares up at larger radii, will control what the accretion disk/AGN will appear as which part of the spectrum will be seen most strongly.

In the case of the sub-mm images produced by EHT, the emission is though to be coming from the ionized plasma closest to the inner edge of the accretion disk (the red region around the black hole in the bottom zoomed-in plot) via synchrotron emission as electrons are accelerated by the magnetic fields of the plasma. This is in contrast to the jet radio emission which is on a larger scale and separation and often extends several galaxy radii which can be seen in e.g. this VLA/VLBI composite radio image. There is some additional information on accretion disks and how they work at this EU Strong Gravity research site and a more technical review article in this conference proceedings.

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  • $\begingroup$ This is great, thanks. Any idea on the second part: Since they're already round, what's that got to do with light bending? $\endgroup$
    – user11801
    Apr 11, 2019 at 19:18
  • $\begingroup$ I've not read the papers (or even the press release) in detail but I thought the proof of light bending was from the size of the black hole in the middle of the accretion disk being larger than you would expect without light bending rather than from the disk image itself ? $\endgroup$
    – astrosnapper
    Apr 11, 2019 at 20:07
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    $\begingroup$ As @Steve Linton says in his answer, the mm waves observed by the EHT are primarily from synchrotron radiation and not from thermal radiation. See iopscience.iop.org/article/10.3847/2041-8213/ab0f43/meta for some discussions and references to more. $\endgroup$
    – Mark Olson
    Apr 11, 2019 at 21:01
  • $\begingroup$ This answer suggests the radiation to be thermal. While there should be thermal radiation from the accretion disk or whatever falling matter, the spectrum is that of synchrotron radiation. Thius is specifically said in the papers about M87 bh. $\endgroup$
    – Alchimista
    Apr 12, 2019 at 7:39

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