So by the time the core of our doomed star collapses all the way down to a back hole, the spin can be ferociously large.
But there’s more. If there is material around the black hole falling in it can change the spin as well. If material fell straight into the black hole, the spin wouldn’t change much.
But if that material comes in at a slight angle, it can actually add to the spin of the black hole, increasing its angular momentum. That gives a kick to the spin rate, bumping it up.
And that brings us back to NGC 1365, located about 60 million light years from Earth. Astronomers used NuSTAR to look at X-rays pouring out of material falling into the black hole there. As that material falls in it heats up to millions of degrees, blasting out X-rays that are easily bright enough to see from Earth with the right equipment. Careful observations allowed astronomers to see these X-rays coming from matter just before it reached the Point Of No Return, at a position called the Innermost Stable Circular Orbit. If it gets any closer, blooop! It falls in, and it’s gone.
This is a pretty amazing finding by the NuSTAR astronomers. It shows that extremely detailed X-rays observations are possible; something that’s very difficult and painstaking to do. It also demonstrates that we can take a pretty close look at black holes and tease out details that were previously not possible to see. By themselves, black holes are invisible, dark, and nearly impossible to observe. But they’re sloppy eaters, and this betrays many of their secrets. Even from 600 million trillion kilometers away.