Oooo! Exciting black hole news: a 70 solar mass black hole found in a binary in our Milky Way?
https://doi.org/10.1038/s41586-019-1766-2
Black holes this big aren't expected to form in environments like our Galaxy where stars contain more of elements other than hydrogen and helium

Stars lose mass throughout their lives, and this limits how big a black hole you can get at the end. The more heavier elements (what astronomers call metals) you have, the more mass you lose. For stars with as many metals as our Sun, you shouldn't end up with something this big!

The orbit of the binary appears to be very circular. This makes it unlikely that the black hole was captured elsewhere. If the measurements are correct, we need to think of a way to form a black hole in situ. This requires some creativity

In the paper, they suggest that if we reduce the amount of mass lost in stellar winds AND if we turn of mass from pair-instability supernova, then we can make the black hole if we start from a 200 solar mass star! This seems a stretch to me

An alternative suggested in the paper is that we actually started out with a triple, and that the 70 solar mass black hole is actually the result of a merger. This may work, there's a lot of freedom here, but I'm not sure if everything can be made to fit. This will be interesting

I like this idea. A binary black hole system in our Galaxy could be a perfect source for @LISACommunity. If the binary would actually merge, it would be a ridiculous source for @LIGO/ @ego_virgo/ @KAGRA_PR! https://twitter.com/duetosymmetry/status/1199820841149771778

Here's the paper free to read on the arXiv https://arxiv.org/abs/1911.11989 
Have we found a 70 solar mass black hole, or is the inferred distance off?

Here's a paper trying to explain how to form this black hole system https://arxiv.org/abs/1911.12357  They find that by turning down the amount of mass lost by stars, they can make a 70 solar mass black hole, but the star is too big to fit in the orbit of LB-1

They conclude that either (i) pair-instability doesn't work as expected, (ii) the black hole is the result of a merger, or (iii) possibly elves?