Orbital Comptonization in accretion disks around black holes

¹ University of Crete, Physics Department, PO Box 2208, 710 03 Heraklion, Crete, Greece
² Foundation for Research and Technology-Hellas, 711 10 Heraklion, Crete, Greece
³ Max-Planck-Institute for Astrophysics, Box 1317, 85741 Garching, Germany

Corresponding author: P. Reig, pablo@physics.uoc.gr

Received: 4 April 2001
Accepted: 7 June 2001

Abstract

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.