The X-ray spectra of accreting Kerr black holes
  The relativistic broad iron lines seen in the X-ray spectra of several active galaxies and Galactic black hole systems are reviewed. Most such objects require emission from within the innermost stable orbit of a non-rotating black hole, suggesting that the black holes are rapidly spinning Kerr holes. We discuss the soft excess, the broad iron line and the Compton hump characteristic of reflection from partially ionized gas and show that they may be a common ingredient in the X-ray spectra of many radiatively-efficient, accreting black holes. Strong gravitational bending of the radiation close to a Kerr black hole can explain the otherwise puzzling spectral variability seen in some objects. The Narrow Line Seyfert 1 galaxies may be among the most extreme objects yet seen. 
Andrew C. Fabian and Giovanni Miniutti