Accretion disc-corona models and X/γ-ray spectra of accreting black holes

It was realized quite early that broad-band X/γ-ray spectra of Galactic black holes (GBHs) can be explained in terms of successive Compton scatterings of soft photons (Comptonization) in a hot electron cloud. The Comptonizing medium was assumed to be thermal with a given temperature, Te, and a Thomson optical depth, τT . The theoretical spectra were computed by analytical (Shapiro, Lightman & Eardley 1976; Sunyaev & Titarchuk 1980) and Monte-Carlo methods (Pozdnyakov, Sobol’ & Sunyaev 1983). The problem with such an approach is that in any specific geometry arbitrary combinations of (τT , Te) are not possible. Both GBHs and Seyfert galaxies show a hardening of the spectra at ∼ 10 keV, which is attributed to Compton reflection (combined effect of photo-electric absorption and Compton down-scattering) of hard radiation from a cold material (White, Lightman & Zdziarski 1988; George & Fabian 1991). Hard radiation, reprocessed in the cold matter, can form a significant fraction of the soft seed photons for Comptonization. The energy balance of the cold and hot phases determine their temperatures and the shape of the emerging spectrum (Haardt & Maraschi 1991, 1993; Stern et al. 1995b; Poutanen & Svensson 1996). The situation becomes more complicated when a notable fraction of the total luminosity escape at energies above ∼ 500 keV. Then hard photons can produce e pairs which will be added to the background plasma. Electrons (and pairs) Comptonize soft photons up to γ-rays and produce even more pairs. Thus, the radiation field, in this case, has an influence on the optical depth of the plasmas, which in its turn produces this radiation. This makes the problem very non-linear. Another complication appears when the energy distribution of particles starts to deviate from a Maxwellian. In the so called non-thermal models, relativistic electrons are injected to the soft radiation field. The steady-state electron distribution should be computed self-consistently, balancing electron cooling (e.g., by Compton scattering and Coulomb interactions) and acceleration, together with the photon distribution. The pioneering steps in solving this problem were done by Stern (1985, 1988) using Monte-Carlo techniques and by Fabian et al. (1986), Lightman & Zdziarski (1987), Coppi (1992) using the method of kinetic equations (see Stern et al. 1995a; Pilla & Shaham 1997; Nayakshin & Melia 1998, for recent developments). Non-thermal model have been used extensively in the end of 1980s and beginning of 1990s for explaining the X-ray spectra of active galactic nuclei (see, e.g., Zdziarski et al. 1990), while recently pure thermal model were preferred, since the data show spectral cutoffs at ∼ 100 keV in both GBHs and Seyferts (Grebenev et al. 1993, 1997; Johnson et al. 1997). However, power-law like