Spectral Index as a Function of Mass Accretion Rate in Black Hole Sourcesстатья
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Дата последнего поиска статьи во внешних источниках: 6 февраля 2017 г.
Аннотация:We present herein a theoretical study of correlations between spectral indexes of X-ray emergent spectra and mass accretion rate (\dot{m}) in black hole (BH) sources, which provide a definitive signature for BHs. It has been firmly established, using the Rossi X-ray Timing Explorer (RXTE) in numerous BH observations during hard-soft state spectral evolution, that the photon index of X-ray spectra increases when \dot{m} increases and, moreover, the index saturates at high values of \dot{m}. In this paper, we present theoretical arguments that the observationally established index saturation effect versus mass accretion rate is a signature of the bulk (converging) flow onto the BH. Also, we demonstrate that the index saturation value depends on the plasma temperature of converging flow. We self-consistently calculate the Compton cloud (CC) plasma temperature as a function of mass accretion rate using the energy balance between energy dissipation and Compton cooling. We explain the observable phenomenon, index-\dot{m} correlations using a Monte Carlo simulation of radiative processes in the innermost part (CC) of a BH source and we account for the Comptonization processes in the presence of thermal and bulk motions, as basic types of plasma motion. We show that, when \dot{m} increases, BH sources evolve to high and very soft states (HSS and VSS, respectively), in which the strong blackbody(BB)-like and steep power-law components are formed in the resulting X-ray spectrum. The simultaneous detections of these two components strongly depends on sensitivity of high-energy instruments, given that the relative contribution of the hard power-law tail in the resulting VSS spectrum can be very low, which is why, to date RXTE observations of the VSS X-ray spectrum have been characterized by the presence of the strong BB-like component only. We also predict specific patterns for high-energy e-fold (cutoff) energy (E fold) evolution with \dot{m} for thermal and dynamical (bulk) Comptonization cases. For the former case, E fold monotonically decreases with \dot{m}, in the latter case, the E fold decrease is followed by its increase at high values of \dot{m}. The observational evolution of E fold versus \dot{m} can be another test for the presence of a converging flow effect in the formation of the resulting spectra in the close vicinity of BHs.