  We have performed Monte Carlo simulations of Compton upscattering of
low-energy photons in an accretion disk around a Schwarzschild black hole. The
photons gain energy from the rotational motion of the electrons in the disk.
The upscattering occurs near the black hole horizon, where the flow velocity of
the electrons approaches the speed of light. We show that this type of
bulk-flow Comptonization can produce power-law X-ray spectra similar to the
ones observed in black-hole X-ray transients in the high/soft state, i.e., a
soft bump dominating the spectrum below ~ 10 keV and a power-law tail with
photon index in the range 2-3. In order to reproduce the observed hard to soft
flux ratio the disk has to have vertical optical depth above ~ 3 at the last
stable orbit. We conclude that the power-law component of the high/soft state
of black-hole transients may be due to an intrinsically cool disk extending all
the way to the hole, without a separate hot plasma component.
