Orbital Comptonization in accretion disks around black holes

We have performed Monte Carlo simulations of Compton upscattering of low-en ergy photons in an accretion disk around a Schwarzschild black hole. The ph otons gain energy from the rotational motion of the electrons in the disk. The upscattering occurs near the black hole horizon, where the ow velocity of the electrons approaches the speed of light. We show that this type of b ulk-flow Comptonization can produce power-law Xray spectra similar to the o nes observed in black-hole X-ray transients in the high/soft state, i.e., a soft bump dominating the spectrum below similar to 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 similar to3 at the last stable orbit. We conclude that the power-law compon ent of the high/soft state of black-hole transients may be due to an intrin sically cool disk extending all the way to the hole, without a separate hot plasma component.