Physics
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010acasn..51..432x&link_type=abstract
Acta Astronomica Sinica, vol. 51, no.4, p. 432-434
Physics
Accretion, Accretion Disks, Black Hole Physics, Hydrodynamics, Ism: Jets And Outflows, Galaxies: Active, Quasars: General, X-Rays: General
Scientific paper
Black hole accretion systems, which are widely believed to be harbored in the central regions of active galactic nuclei (AGNs), low-luminosity AGNs (LLAGNs) as well as some X-ray binaries (XRBs), are the key physical processes to understand their observational phenomena, like spectral energy distribution, radiative variability, etc. In this thesis, we focus on the hot accretion flow models, including advection-dominated accretion flow (ADAF) and luminous hot accretion flow (LHAF). These models are the foundations to explain the observations of LLAGNs and XRBs in hard state. In Chapter 1, a detailed description of the background is presented. First the astrophysical black holes and the systems in which they reside are discussed. Then, an extensive discussion on the accretion process is presented. The basic concepts, 4 well-known accretion models and the mechanism of the transition between ADAF and standard thin disk are focused on. After this, we further describe the properties of ADAF - the basic model of this thesis, e.g., the dynamics, the radiative processes and several recent progresses: outflow, direct turbulent heating to the electrons, as well as LHAF at relatively high accretion rate. In Chapter 2, the influences of outflow on the dynamics of inflow are explored. As indicated through observations (e.g., towards the Galactic center), theoretical researches and (magneto-) hydrodynamical simulations, outflow is a common phenomenon in accretion systems. However, most researches in this field, especially when aiming at explaining/fitting observational data, incline to only include the mass loss due to the existence of outflow, while all the other effects like the angular momentum transport are totally neglected. This obviously conflicts with the results from simulations. Since outflow is not fully understood currently, we here parameterize its properties. Our results are shown as follows: (1) under current status of observations and theories, it is acceptable to only include its mass loss contribution, the derived properties (e.g., density, temperature) are accurate within a factor of 2˜3; (2) outflow's other dynamical influences (angular momentum and/or internal energy transport) can never be neglected, provided that these highly deviate from those in inflow. Besides, compared with all the previous work, our new approach also has the advantage of its potential applications in exploring the dynamics and radiation of outflow itself. In Chapter 3 and 4, another important mechanism -- Compton scattering -- in hot accretion flows is studied. Being tenuous and hot, the main radiative cooling processes in ADAF are synchronization, bremsstrahlung and their inverse Comptonization. The Comptonization will dominate at relatively high accretion rates. We note that almost all the previous works are based on the one-zone approximation, in which only the local Comptonization (in vertical direction) is considered. On the other hand, we note that ADAF is also optically thin in the radial direction, and the gradient of the electron temperature is high, thus the Comptonization in radial direction (global Comptonization) should also be important. It is found that, although the momentum transport due to this global Compton scattering is negligible, its energy transport is significant. Based on the treatment of the Comptonization procedure, we separate our work into two steps. First in Chapter 3, the global Comptonization effect in hot accretion flows is generally investigated. Due to the complexity in solving the radiative transfer equation in the radial direction, we apply a simplified treatment to calculate the global Compton scattering in the radial direction. The results are: (1) the inner regions of the hot accretion flow are cooled down, while the outer regions are heated up, the dividing radius is approximately 5×10^3rs, where rs is Schwarzschild radius; (2) the upper limit of accretion rate in ADAF is reduced, while the radiative efficiency should be significantly increased, due to the strong global Compton scattering in hot accretion flows; (3) the global Compton heating effect in the outer regions may cause the "oscillation" of the accretion flow in AGN between active and non-active phases. The duration of the active phase approximately equals to the accretion timescale at the virial radius, while the duration of the non-active phase may be comparable to the cooling timescale at the virial radius. Subsequently in Chapter 4, the more accurate Monte Carlo simulations are used to uniformly deal with the Compton scattering process and explore the Compton cooling effect in the inner regions (r≲300rs) of the hot accretion flows. The results by using this approach are consistent with those in Chapter 3. Besides that, it is found that the radiative efficiency is increased by a factor of 5 at 0.05dot{M}_{Edd}, much higher than the expected; the spectral shape is also modified due to the existence of global Comptonization. We then discuss the contribution of the outflowing material to the observed spectrum. We find that the temperature and column density of outflow can partly help to explain one of the major difficulties in accretion fields, i.e., the temperature and optical depth from observational fittings deviate from what are predicted by ADAF theories. We also confirm the previous analysis (Yuan 2001, 2003) that the inner regions of hot accretion flow is thermally instable. One consequence is that the flow will collapse to form a thin disk. The other possibility predicted by LHAF is that the hot accretion flow will be filled with cold clumps/clouds. Disappointedly, we cannot rule out any of these two possibilities at present. The latter, namely two-phase accretion mode, could explain the steep power-law state in XRBs. In Chapter 5, a brief discussion of conceived researches related to this thesis is presented.
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