General-relativistic magnetohydrodynamics simulations of black hole accretion disks: Dynamics and radiative properties

The goal of the series of studies in this thesis is to understand the black hole accretion process and predict its observational properties. The highly non-linear process involves a turbulent magnetized plasma in a general relativistic regime, thus making it hard to study analytically. We use numerical simulations, specifically general relativistic magnetohydrodynamics (GRMHD), to construct a realistic dynamical and radiation model of accretion disks. Our simulations are for black holes in low luminous regimes that probably possesses a hot and thick accretion disk. Flows in this regime are called radiatively inefficient accretion flows (RIAF). The most plausible mechanism for transporting angular momentum is turbulence induced by magnetorotational instability (MRI). The RIAF model has been used to model the supermassive black hole at the center of our Milky Way galaxy, Sagittarius A* (Sgr A*). Owing to its proximity, rich observational data of Sgr A* is available to compare with the simulation results. We focus mainly on four topics. First, we analyse numerical convergence of 3D GRMHD global disk simulations. Convergence is one of the essential factors in deciding quantitative outcomes of the simulations. We analyzed dimensionless shell-averaged quantities such as plasma beta, the azimuthal correlation length (angle) of fluid variables, and spectra of the source for four different resolutions. We found that all the variables converged with the highest resolution (384x384x256 in radial, poloidal, and azimuthal directions) except the magnetic field correlation length. It probably requires another factor of 2 in resolution to achieve convergence. Second, we studied the effect of equation of state on dynamics of GRMHD simulation and radiative transfer. Temperature of RIAF gas is high, and all the electrons are relativistic, but not the ions. In addition, the dynamical time scale of the accretion disk is shorter than the collisional time scale of electrons and ions, which makes the gas have two temperatures. We assumed that the temperature ratio of the ions and electrons Tp/Te is constant and constructed a new Synge-type equation of state that takes the effect of two temperature fluid as well as variable adiabatic index caused by the non-relativistic to relativistic transition of the particles into account. We found that the effect of the Synge-type equation of state on the dynamical model is negligible since the temperature variation in the flow is small enough to keep the effective adiabatic index in time and space approximately constant. The spectra are not affected by the equation of state either. Thirdly, we studied effects of accretion rate on radiative properties of a black hole accretion disk. We used GRMHD simulation and general relativistic ray-tracing to formulate the relation between accretion rate and image size, as well as flux. The result will be compared to the expected change in emission of Sgr A* due to the interaction with the approaching gas cloud G2. Finally, we constructed a dynamical and radiative model of tilted black hole accretion disks. Rotational axis of accretion disks are not necessarily aligned with spin of black holes. When they are not aligned, deformation of the accretion disks (warps, twists, or emergence of new structure) are expected due to the gravito-magnetic effect. We initially tilted otherwise equilibrium RIAF disks, seeded with a weak magnetic field, and evolved. We found that m = 2 standing waves show up for radius < 6GMBH/c 2 with ratio of the mode amplitude of m=2 and m=0 2-5 times larger than that of untilted disks. Shocks produced along the waves heat up the gas and raise the temperature at inner radii. Resultant spectra of tilted disks have a substantial increase in flux of the order of 1-2 magnitude for 15° and 30° tilted disks in infrared and X-ray. The result assures the tilt is one of the fundamental parameters in modelling black hole accretion disks.