Adiabatic Accretion Flows Research Articles

Parameter space study of the magnetohydrodynamic accretion flows around compact objects

We solve the magnetohydrodynamic (MHD) equations governing axisymmetric flows around compact objects and found all possible classes of solutions for non-relativistic adiabatic accretion flows. We divide the parameter space in terms of these classes. We study the possibility of the formation of the MHD shock waves and show how the strength of the shocks depends on the flow parameters. We also show regions of the parameter space where the shock conditions are not satisfied and therefore the shocks may oscillate. These solutions are astrophysically interesting as they could give rise to quasi-periodic oscillations seen in hard X-rays.

Non-axisymmetric instabilities in shocked adiabatic accretion flows

We investigate the linear stability of a shocked accretion flow on to a black hole in the adiabatic limit. Our linear analyses and numerical calculations show that, despite the post-shock deceleration, the shock is generally unstable to non-axisymmetric perturbations. The simulation results of Molteni, Toth & Kuznetsov can be well explained by our linear eigenmodes. The mechanism of this instability is confirmed to be based on the cycle of acoustic waves between the corotation radius and the shock. We obtain an analytical formula to calculate the oscillation period from the physical parameters of the flow. We argue that the quasi-periodic oscillation should be a common phenomenon in accretion flows with angular momentum.

Isothermal Shock Formation in Nonequatorial Accretion Flows around Kerr Black Holes

We explore isothermal shock formation in nonequatorial, adiabatic accretion flows onto a rotating black hole, with possible application to some active galactic nuclei (AGNs). The isothermal shock jump conditions as well as the regularity condition, previously developed for one-dimensional flows in the equatorial plane, are extended to two-dimensional nonequatorial flows to explore possible geometric effects. The basic hydrodynamic equations with these conditions are self-consistently solved in the context of general relativity to explore the formation of stable isothermal shocks. We find that strong shocks are formed in various locations above the equatorial plane, especially around a rapidly rotating black hole with the prograde flows (rather than a Schwarzschild black hole). The retrograde flows are generally found to develop weaker shocks. The energy dissipation across the shock in the hot nonequatorial flows above the cooler accretion disk may offer an attractive illuminating source for the reprocessed features, such as iron fluorescence lines, which are often observed in some AGNs.

Global solutions of adiabatic accretion flows with isothermal shocks in Kerr black hole geometry

This paper presents global solutions of adiabatic accretion flows with isothermal shocks in Kerr black hole geometry. It is known that in the previously studied cases, where the flow including the shock is either entirely adiabatic or entirely isothermal, there can be no more than one stable shock solution, and the solution can only be of α–x type. However, the solution topology in the present case shows remarkable new characteristics: for the same flow parameters there can be two stable shock solutions satisfying physical boundary conditions, and the solution can be of three types, namely α–x, x–α and α–α type. In addition, shocks in the present case occur for a parameter region different from that for Rankine–Hugoniot shocks. These results greatly increase the possibilities of shock formation in astrophysical flows. It is also significant that the effects of frame-dragging of a rapid Kerr black hole on the shock formation are discovered. Finally, a brief comparison is made between shocked inviscid flows and two types of shock-free viscous flows, namely those of Shakura & Sunyaev and Narayan & Yi, and some comments are made about the fact that numerous authors who have studied transonic global solutions of accretion flows have found no shocks.

Global solutions of adiabatic accretion flows with isothermal shocks in Kerr black hole geometry

Global solutions of adiabatic accretion flows with isothermal shocks in Kerr black hole geometry

Isothermal Shocks in Adiabatic Black-Hole Accretion Flows

This paper presents global solutions of adiabatic accretion flows with isothermal shocks in the potential of a non-rotating black hole. It is known that in the previously studied cases, where flow including shock is either entirely adiabatic or entirely isothermal, there can be no more than one stable shock solution, and the solution can only be of the α-x type; however, the solution topology in the present case shows new remarkable characteristics: for the same flow parameters there can be two stable shock solutions satisfying the physical boundary conditions, and the solution can be of three types, namely α-x, x-α, and α-αtype. In addition, shocks in the present case occur for a parameter region that is different from that for Rankine-Hugoniot shocks to occur. These results greatly increase the possibilities of shock formation in astrophysical flows. It is also noted that in the present case the efficiency of energy release at the shock can be as high as a few percent. Finally, a brief comparison is made between shocked inviscid flows and two types of shock-free viscous flow, namely that of Shakura and Sunyaev (1973, AAA 09.066.049) and that of Narayan and Yi (1994, AAA 61.064.083).

On the morphology of accretion flows with small, non-zero specific angular momentum

The morphology of adiabatic accretion flows with small non-zero specific angular momentum has been investigated in the axisymmetric and nonviscous limit. For an initial state characterized by a Bondi flow with the specific angular momentum distributed with respect to polar angle, a travelling shock wave forms which propagates more rapidly in the equatorial plane than in the plane perpendicular to it resulting in the formation of a hot torus. In cases where the incoming flow is restricted to lie near the equatorial plane, a strong wind forms directed away from this plane with the tendency for the formation of a nonsteady shock structure. As the height of the incoming flow is increased it is found that the resultant wind weakens. The parameter regime which delineates accretion flows characterized by a travelling shock and a nearly standing shock structure is presented.

Dynamical instability of standing shock waves in adiabatic accretion flows and wind flows

Dynamical instability of standing shock waves in adiabatic accretion flows and wind flows