Astronomers detected a supernova like explosion some 0.3 bLY away. And then a periodic blip of xray every 2 minutes running for 2 years without reduction in magnitude from that point.
They figured out it's a blackhole feeding a star million sized our sun. And from the blip worked out "spin" of blackholes.
And from spin of many other blackholes they finally expect to figure out whether blackholes grow by feeding on galactic stars. Or IF they grow mostly from merging with other blackholes. That's still unanswered.
Excellent 10 min presentation explaining a lot of things.