On the time lags in Galactic black holes

The origin of time lags between hard and soft photons observed in Galactic black holes remains a mystery even 10 years after their discovery (Miyamoto et al. 1988). Since it is generally believed that Comptonization is responsible for spectral formation, these lags are attributed in some models to the delays due to scattering in a Compton cloud. The Fourier frequency dependence (similar to 1/f) of the lags and their amplitudes (up to similar to 0.1 s) require then that a hot cloud extends over 10(4) gravitational radii from the central black hole and has an inhomogeneous density distribution, n(r) proportional to 1/r (Kazanas, Hua & Titarchuk 1997). The existence of such a cloud is highly improbable due to energetic problems. In the present paper, we argue that the origin of time lags is spectral evolution. We assume that the X/gamma-rays in accreting black holes are produced in compact magnetic flares at the surface of cold accretion disks. If the spectrum of the flare evolves from soft to hard, the time delays appear between the arrival of soft and hard photons. A possible reason for the spectral evolution is a decrease with time of the feedback factor due to, e.g., inflation of the magnetic loop configuration and/or increase of the bulk velocity of the emitting hot plasma.