  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.
