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.