Studies On The Bimodal Black Hole Accretion Discs

The thesis herein briefly reviews the theory of accretion onto a black hole and its progress made in recent years. Four known accretion models, namely the standard thin disk (SSD), the SLE (Shapiro, Lightman & Eardley) disk, the Slim disk, and the Advection-Dominated Accretion Flow (ADAF), are discussed, and a unified description of the above four models at a fixed radius R is presented. The ADAF around a black hole is a popular model in recent years. Its basic equations, self-similar solutions, global transonic solutions, and the dynamical properties have been introduced. Bimodal black hole accretion disk-SSD-ADAF has been quite successfully applied to black hole X-ray binaries and active galactic nuclei. There are four currently competing models of mechanisms that lead to an SSD-ADAF transition, i.e., strong ADAF principle, evaporation, Honma mechanism and thermal instability in the inner SSD zone. An introduction to these mechanisms is also presented.Transition radius Rtr was inferred from a plausible assumption that it dependson accretion rate M . The result from different mechanism is quite different. In this thesis, we dedicate to make a further investigation on the mechanism that a thermal instability of a radiation pressure-supported SSD triggering the flow to jump from a SSD state to an ADAF state. In Chapter two, considering the energy equation, we study this mechanism, and find that a thermal instability of a radiation pressure-supported SSD can trigger such a transition. Sonic point (Rs) and Rtr aredetermined by parameters M , M , j and a. In Chapter three, we investigateglobal dynamics of ADAF abound black holes, taking the bremsstrahlung radiation as a local radiative cooling mechanism, and simply assuming that the accretion flow is made of one temperature plasma. We adopt a bridge formula for radiation in our calculation, which is valid in both optically thin and optically thick regimes. The variables radial velocity v , sound speed cs , angular momentum l, relativethickness H/R, optical depth , temperatureT, and advection factorQadv/Qvis as afunction of R indicate that the inner zone the accretion flow satisfies the ADAF condition and the outer region satisfies the SSD condition. These results suggest that a smooth transition of SSD-ADAF is possible, and Rtr is around 10 Rg, being close tothe central black hole. Two-temperature ADAF is a more general and more self-contained scenario. In Chapter four, we investigate the transition of the two-temperature SSD-ADAF. We show that a smooth SSD-ADAF transition is possible for black hole accretion flows with high viscosity and non-zero radiative cooling (we consider rigorously two-temperature plasma with comprehensive radiative cooling mechanisms including bremsstrahlung, synchrotron, and their Comptonization.), and the transition radius is around 10Rg.Based on our results, we conclude that the thermal instability of a radiation pressure-supported SSD can trigger the SSD-ADAF transition, and the transition radius is close to the central black hole.