First black hole ever detected is more massive than we thought

Interesting. The home page was last updated on 08 February 2020 and of course alot has happened in West Virginia since then. A press conference presentation indicates funding through to this year. Here is the author's home page at West Virginia University and he has a very impressive CV. To sign up for a newsletter is easy with a Google account and I see the last entry as :

Hmmm. It seems his professional email address per his home page is zbetienne-[at]-mail.wvu.edu and I have just penned an inquiry ! Thanks for bringing this to my attention.

Spin is always going to be a parameter for BH systems study for at least these reasons :

- unless there has been an accident of history all BH systems will have it ie. incredibly unlikely that they were created without any angular momentum of the ingoing components.

- there's only three observables for BHs : mass, spin and electric charge. I doubt if any ( excess ) charge would ever be significant enough for us to detect an effect at distance. That's not to say there wouldn't necessarily be some electromagnetic signal from whatever surrounds the BHs during a merger. 

- yet another opportunity to test Einstein's general relativity.

Cheers, Mike.

That's one way to look at it. The photons down deep in the gravitational well of a rotating black hole get an effective repulsion component from gravitomagnetism ( they have a mass equivalent ) acting upon them. Thus they can probe the region closer to the hole without getting captured ( compared to the non rotating case ). 

Short Answer : It approaches half the Schwarzschild radius.

Long Answer : If one defines an event horizon as where a coordinate singularity occurs in the radial component of a chosen metric, then Kerr black holes have two event horizons. There's a quadratic involved and thus two solutions where the relevant expression becomes infinite. For the outer one, mathematically it looks like :

r = M + SQRT[M² - (L/M)²]                               ( where G = c = 1 )

{ The inner horizon is at r = M - SQRT[M² - (L/M)²] and I really don't know what that physically means. }

L is the hole's angular momentum and M is the mass of the hole. The second square-root term goes to zero when L -> M², thus r -> M.

{ If L -> 0 then r -> 2M which is the Schwarzchild radius. It is proportional to M. The units work out to a radius of  ~3km per solar mass. }

{ The radii I speak of are measured in the equatorial plane of the hole ie. defined as the plane perpendicular to the angular momentum vector. }

But the niggly bit is what happens when L > M² as that makes for a negative expression under the square root and no real solutions for r. Does that mean there are no event horizons and the central singularity is 'naked' ? Opinions differ, where for instance Roger Penrose has coined his cosmic censorship principle that prevents a singularity from being seen. But I understand that hasn't been proven yet. ;-)

Another interesting point about frame dragging means that if you start descending into the hole with an orbit in the opposite sense to the hole's rotation, once you get sufficiently close in you just have to 'go with the flow' as it were and rotate with the hole.

Cheers, Mike.

( edit ) I get my information from a ripper of a book called Differential Forms and the Geometry of General Relativity by Tevian Dray. It bypasses all the tensor kerfuddling and gets right into the meaning of the various metrics. You just have to accept the solutions to the GR equations as discovered and get on with the job of interpretation. Since I'm not in the business of finding new solutions then one can avoid the gorrible tensor mess. Differential geometry has been called, tongue in cheek, the study of objects that are invariant with respect to a change in notation. :-) 

( edit ) Metric means the method by which displacements in spacetime are measured in the limit of small 'distances'. Think of it as a rule to determine how to take a tiny step. If you want to go a long way then you add up ( mathematically integrate ) lots of tiny steps to determine a distance travelled along some path. Geodesics are the paths of freely falling bodies which have the property that they minimise total 'timelike distance' travelled : the 'shortest' lines are taken by light. So the idea of gravitational forces converts to the demonstration of geodesic paths in non-Euclidean or 'curved' geometries. The above analysis looks at where the metric expression 'blows up' and that's a challenge to interpret.