Timing and Spectral Properties of X-Ray Emission from the Converging Flows onto a Black Hole: Monte Carlo Simulations

Abstract

We demonstrate that an X-ray spectrum of a converging inflow (CI) onto a black hole is the sum of a thermal (disk) component and the convolution of some fraction of this component with the Comptonization spread (Green's) function. The latter component is seen as an extended power law at energies much higher than the characteristic energy of the soft photons. We show that the high-energy photon production (source function) in the CI atmosphere is distributed with the characteristic maximum at about the photon bending radius, 1.5rS, independently of the seed (soft) photon distribution. We show that high-frequency oscillations of the soft photon source in this region leads to the oscillations of the high-energy part of the spectrum but not of the thermal component. The high-frequency oscillations of the inner region are not significant in the thermal component of the spectrum. We further demonstrate that Doppler and recoil effects (which are responsible for the formation of the CI spectrum) are related to the hard (positive) and soft (negative) time lags between the soft and hard photon energy channels, respectively.