Stability of Accretion Flows And RadiativeHydrodynamics Around Rotating Black Holes
Abstract
In the case of cold accretion disk, coupling between charge neutral gas and magnetic field is too weak such that the magnetorotational instability will be less effective or even stop working. In such a situation it is of prime interest to investigate the pure hydrodynamic turbulence and transport phenomenon. As the Reynolds number increases, the relative importance of the nonlinear term in the hydrodynamic equation increases and in the case of accretion disk where molecular viscosity is too small the Reynolds number is large enough for the nonlinear term to bring new effects. We investigate a scenario, the ‘weakly nonlinear’ evolution of amplitude of linear mode when the flow is bounded by two parallel walls. The unperturbed flow is similar to plane Couette flow but with Coriolis force included in the hydrodynamic equation. Although there is no exponentially growing eigenmode, due to selfinteraction the least stable eigenmode will grow in an intermediate phase. Later on this will lead to higher order nonlinearity and plausible turbulence. Although the nonlinear term in the hydrodynamic equation is energy conserving, within the weakly nonlinear analysis it is possible to deﬁne a lower bound of the energy needed for flow to transform to turbulent phase. Such an unstable phase is possible only if the Reynolds number ≥ 103−4. In Chapter2 we set up equation of amplitude for the hydrodynamic perturbation and study the effect of weak nonlinear evolution of linear mode for general angular momentum distribution, where Keplerian disk is obtained as a special case.
As we know that to explain observed hard Xrays the choice of Keplerian angular momentum profile is not adequate, we consider the subKeplerian regime of the disk. In Chapter3 we assume that the cooling mechanism is dominated by bremsstrahlung process (without any strict knowledge of the magnetic field structure).We show that in a range of ShakuraSunyaev viscosity 0.2 ≥ α ≥ 0.0005, flow behavior varies widely, particularly by means of the size of disk, efficiency of cooling and corresponding temperatures of ions and electrons. We also show that the disk around a rotating black hole is hotter compared to that around a Schwarzschild black hole, rendering a larger difference between ion and electron temperatures in the former case. We finally reproduce the observed luminosities(L) of two extreme cases—the underfed AGNs and quasars and ultraluminous Xray sources at different combinations of mass accretion rate, ratio of specific heats, ShakuraSunyaev viscosity parameter and Kerr parameter.
In Chapter4 we investigate the viscous two temperature accretion disk flows around rotating blackholes. We describe the global solution of accretion flows, unlike that in Chapter3, with a subKeplerian angular momentum profile, by solving the underlying conservation equations including explicit cooling processes selfconsistently. Bremsstrahlung, synchrotron and inverse comptonization of soft photons are considered as possible cooling mechanisms. We focus on the set of solutions for subEddington, Eddington and superEddington mass accretion rates around Schwarzschild and Kerr black holes with a Kerr parameter 0.998. We analyse various phases of advection–general advective paradigm to radiatively inefficient paradigm. The solution may potentially explain the hard Xrays and γrays emitted from AGNs and Xray binaries. We also compare the solutions for two different regimes of viscosity. We finally reproduce the observed luminosities of the underfed AGNs and quasars, ultraluminous Xray sources at different combinations of input parameters such as mass accretion rate and ratio of specific heats.
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 Physics (PHY) [336]
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