Bulk Motion Comptonization - a sure sign of Black Holes
  Bulk motion Comptonization utilizes properties of both matter and radiation close to the horizon of a black hole. Computation with these considerations produce hard tails of energy spectral slope $\sim 1.5-1.7$. These are the most direct evidence of the horizon of a black hole. We argue that even in presence of winds and outflows this property is not likely to change as winds are negligible in soft states. 
Sandip K. Chakrabarti and Indranil Chattopadhyay