Scientists detect rare crash of two mismatched black holes for the first time ever

An observation at high significance, meaning the mass difference likely is real. Despite that they see a large in-plane (I think) spin (which was easier than in equal mass mergers) of the larger black hole, they can't reject that it was a merger between pristine black holes. Else the hierarchical merger - the larger black hole already being a merger result - looks interesting! A birth place in a dense cluster with some trinary star systems is a possibility.

The mass asymmetry meant odd overtones in the gravity wave spectra, so they could test general relativity theory further than ever before - it still works perfectly well!
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I note this from the article, "The newly announced discovery involved objects about 2.4 billion light-years away, Fishbach said, with one black hole about eight times the mass of our sun and the other about 30 times the mass of our sun. "

However, another report indicates the distance is spread 1.9 to 2.9 billion light-years, LIGO and Virgo detectors catch first gravitational wave from binary black hole merger with unequal masses, "GW190412 was observed by both LIGO detectors and the Virgo detector on 12th of April 2019, early during the detectors' third observation run O3. Analyses reveal that the merger happened at a distance of 1.9 to 2.9 billion light-years from Earth. The new unequal mass system is a unique discovery since all binaries observed previously by the LIGO and Virgo detectors consisted of two roughly similar masses. "

Distance is important when determining masses, etc.