Observational evidences for spinning black holes: A proof of general   relativity for spacetime around rotating black holes
  Since it was theorized by Kerr in 1963, determining the spin of black holes from observed data was paid very little attention until few years back. The main reasons behind this were the unavailability of adequate data and the lack of appropriate techniques. In this article, we explore determining/predicting the spin of several black holes in X-ray binaries and in the center of galaxies, using X-ray and gamma-ray satellite data. For X-ray binaries, in order to explain observed quasi-periodic oscillations, our model predicts the spin parameter of underlying black holes. On the other hand, the nature of spin parameters of black holes in BL Lacs and Flat Spectrum Radio Quasars is predicted by studying the total luminosities of systems based on Fermi gamma-ray data. All sources considered here exhibit characteristics of spinning black holes, which verifies natural existence of the Kerr metric. 
Banibrata Mukhopadhyay (IISc), Debbijoy Bhattacharya (IUCAA/Manipal),   P. Sreekumar (ISAC)
astro-ph.HE astro-ph.CO gr-qc