Heaviest pair of black holes ever seen weighs 28 billion times more than the sun

I am left wondering what the merger of 2 such massive black holes, located 90 million light years away, would look like from Earth. Would we even notice it without a telescope? Or, would it make a naked-eye spectacle in Earth's sky?
The SMBH diameter is about 1106 or so AU in size and angular size about 4.005E-4 mas compared to the paper looking at areas about 1.3 to 1.5 arcsecond size. Plenty to geek out on in the paper cited. How long does it take for a 28 billion solar mass black hole to form? 1 year, 1 trillion years?
I think the formation of a merged black hole would occur pretty rapidly, and release a very large amount of energy in the process. But, that is based on smaller black holes. Maybe extremely large ones do it more slowly? And, I have not idea how much of that energy would be immediately radiated away and now much would be trapped inside the new event horizon. And, at 90 million light years distance, I am wondering how bright it has to be for it to be seen from Earth with the naked eye.

I also wonder if "quasars" could be merging black holes, rather than single black holes feeding on stars at extreme rates. I don't think we can resolve quasars in any detail, so their explanation is largely theoretical, anyway.
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The report and others now out indicate the merger is *stalled*, some reports indicate for some 3 billion years or more.

"A stalled merger The mass of the system's two black holes is so great that the team thinks it would take an exceptionally large population of stars around them to bring the supermassive black holes close together. As this has been happening, however, the energy leached from the binary has been flinging matter away from their vicinity. This has left the center of B2 0402+379 bereft of stars and gas close enough to the binary to leach energy from it. As a result, the progress of these two supermassive black holes toward each other has stalled as they approach the final stages before a merger.", space.com report.

"The team used data from the Gemini North telescope in Hawai'i, one half of the International Gemini Observatory operated by NSF's NOIRLab, to analyze a supermassive black hole binary located within the elliptical galaxy B2 0402+379. This is the only supermassive black hole binary ever resolved in enough detail to see both objects separately, and it holds the record for having the smallest separation ever directly measured—a mere 24 light-years. While this close separation foretells a powerful merger, further study revealed that the pair has been stalled at this distance for over three billion years, begging the question: What's the holdup?", https://phys.org/news/2024-02-astronomers-heaviest-black-hole-pair.html

Perhaps such SMBH take longer than the age of the Universe to form :)
Yes, the black hole merger stalling theory seems to create more problems for the BBT. But, I don't think we understand the merging process well enough (yet?) to draw any firm conclusions.

The apparent fact that there are no new, close quasars in our vicinity seems to suggest that something that happened previously is now not so common (or even possible ?), so maybe black hole mergers only stall above a specific mass level, and maybe mergers below that mass threshold, but still quite large, cause quasars?

Just "spit-ballin'" with the concepts.

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