Rotation of a black hole

A Kerr black hole has a theoretical rotation component due to conservation of angular momentum. It cannot be measured directly as we cannot observe anything occuring inside the event horizon. I doubt it's meaningful to talk about rotataion with respect to the event horizon. For discussion, see
http://science.howstuffworks.com/black-hole.htm/printable

 

I heard about that. In an x-ray binary in which a black hole is believed to be the compact object, it has been suggested that the strength of an ultrasoft x-ray component is directly related to the black hole's spin.
Check out section 8.2 of the following:
http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.astro.44.051905.092532

 

Good link, maria, but, I am unconvinced the authors have derived an effective way to derive the intrinsic spin of a black hole.