  An accreting black hole is, by definition, characterized by the drain.
Namely, the matter falls into a black hole much the same way as water
disappears down a drain - matter goes in and nothing comes out. As this can
only happen in a black hole, it provides an unique way to see it. The accretion
proceeds almost in free fall close to the black hole horizon. In this paper we
calculate (by using Monte -Carlo simulations) the specific features of X-ray
spectra formed as a result of upscattering of the soft (disk) photons in the
converging inflow (CI) within about 3 Schwarzschild radii of the black hole.
The full relativistic treatment has been implemented to reproduce these
spectra. We show that spectra in the soft state of black hole systems can be
described as the sum of a thermal (disk) component and the convolution of some
fraction of this component with the CI upscattering spread function. The latter
boosted photon component is seen as an extended power-law at energies much
higher than the characteristic soft photons energy. We demonstrate the
stability of the power spectral index (alpha= 1.8) over a wide range of the
plasma temperature 0-10 keV and mass accretion rates (higher than 2 in
Eddington units). We also demonstrate that the sharp high energy cutoff occurs
at energies of 200-400 keV which are related to the average rest energy of
electrons impinging upon the horizon. The spectrum is practically identical to
the standard thermal Comptonization spectrum when the CI plasma temperature is
getting of order of 50 keV (hard state of BHS). Also, the change of spectral
shapes from the soft to the hard X-ray state is clearly to be related with the
temperature of the bulk flow. These Monte-Carlo simulated CI spectra are then a
inevitable stamp of the BHS.
