My understanding of the abstract is (I'm not an astronomer):

TON 618 is an active SMBH ("hyperluminous, broad-absorption-line, radio-loud quasar" [0]).

The SMBH they found has a smaller mass [2], but it is passive (= much darker).

From the abstract:

"Outside the local Universe, measurements of MBH are usually only possible for SMBHs in an active state: limiting sample size and introducing selection biases. Gravitational lensing makes it possible to measure the mass of non-active SMBHs." [1]

[0] https://en.wikipedia.org/wiki/TON_618

[1] https://academic.oup.com/mnras/article-abstract/521/3/3298/7...

[2] "it could be a supermassive black hole equivalent to 13 billion suns: 1.3±0.6)×10^10 M" https://en.wikipedia.org/wiki/Abell_1201_BCG

edit the bbc article got it wrong withe 30 billion times the size and it supposed to be 30 billion solar masses see [1] or the arxiv article [2]

The bbc article says 30 billion times the size while TON618 is 66 billion times the mass.

Wikipedia says TON618 has a Schwarzschild radius of 1,300 AU (390 billion km in diameter) vs teh sun's 1.3927 million km diameter. Which makes TON618 ~280k bigger than the sun.

So if my math is right that makes the Ultramassive balck hole in the bbc article ~107k larger than TON618.

edit give the R = 3M relation for black holes twice the radius means twice the mass. So 107K times the radius means 107k times the mass.

The Ultramassive balck hole in the bbc article would have ~2354000 billion solar masses.

edit give the above I want to fact check BBC's 30 billion times the size vs 30 billion solar masses since the later seems more reasonable.

edit 30 billion solar masses is what is given directly by durham and arxiv[2]

[1] https://www.durham.ac.uk/news-events/latest-news/2023/03/lig... [2] https://arxiv.org/abs/2303.15514

I mean...There are others bigger than that with Phoenix A, but these are quasars and very visible, even if billions of LY away.

https://en.wikipedia.org/wiki/Phoenix_Cluster#Supermassive_b...

It seems the bigger tact is HOW it was discovered using gravitational lensing that may open up the ability to see otherwise "non-active" blackholes, even supermassive ones that otherwise dont have accretion discs or other forms of waves (radio, light or otherwise) that would make them directly observable.

https://arxiv.org/pdf/2303.15514.pdf

I am also not an astronomer, so dont take my word for it. Just seems that it may be a new method that may open up the doors to a more discoveries that may make things like this a bit more....common.

TON618 and Phoenix A even are somewhat outliers from a discovery standpoint it seems. And frankly the sizes, distances etc are basically incomprehensible to me.