Dynamical Properties of Oscillating, Viscous, Transonic Accretion Disks around Black Holes

We investigate the time evolution of sub-Keplerian transonic accretion flow onto a nonrotating black hole using axisymmetric viscous hydrodynamic simulations. We simulate the accretion flow using boundary values from semianalytical analysis and set up three different models. Two of the models do not predict accretion shocks from the semianalytic analysis, while one of them does. We also consider radiative cooling along with viscosity in the simulation. Our two-dimensional simulation deviated from the one-dimensional semianalytical solution and admitted shocks in all three models. Viscous dissipation tends to push the shock front outward, and radiative cooling will push it in. Additionally, gravity is attractive. Depending on the competing strengths of all three processes, it may trigger shock oscillation. Different rates of angular-momentum transport in various layers may trigger eddies, which will enhance the shock oscillation. We show that any simple power law cannot approximate these solutions. We find that hot and higher angular-momentum flow requires higher viscosity to produce oscillatory shocks. From the temporal variation of the luminosity, shock oscillations generate quasiperiodic oscillations in the range of sub-Hertz to a few Hertz frequencies if a 10 M ⊙ black hole is assumed.