Black hole accretion with small angular momentum

 

Pure spherical accretion is physically quite improbable, but the low angular momentum accretion could explain the physical processes in low luminous sources (low luminous galactic centres or binary systems in quiescent state). The study of spherically symmetric situation perturbed by imposing angular momentum to the gas small compared to the Keplerian values is thus of physical interest and could be related to the variability of low luminous sources.

We show the example of evolution of gas, which is initially distributed according to the Bondi solution and angular momentum depending on the polar angle is imposed. The pseudo-Newtonian potential is assumed and the black hole is a Schwarzschild one. Click on the movie to see how the slowly rotating quasi-spehrical flow evolves. (Simulations with the code ZEUS. Author: Petra Sukova,2015)

 

The simulations yield the shock front unstable for a subset of parameters which leads to oscillation of the shock front around the position given by the steady solution with frequency depending on the distance to the center. The evolution of the flow with changing angular momentum and repeating creation and disappearance of the shock front leads to the hysteresis loop. In the video below, the Mach number is denoted as u/a by orange line, density rho by purple line and angular momentum L by yellow line.

 

In our recent work, we extend the study of oscillating shock bubble in the flow with low angular momentum and study the infuence of the adiabatic index (γ) on it. The initial conditions in the simulations are similar to those used in previous work (Sukova & Janiuk 2015). We compute sets of models, spanning the whole parameter space which determines the location of shock and sonic positions during the accretion of the flow. We chose a wide range of values for γ ( γ =1.02 to γ = 5/3 ) for a broader understanding of the microphysics of the flow and how accretion of matter is affected by the changing degrees of freedom and thermal properties of the constituents of the flow.

Our simulation shows that increase of the adiabatic index prevents the shock from accretion or blows the shock to expand, while the shock bubble is steady for lower γ. The amount of pressure created in the flow during evolution works against shock and that amount depends on the value of γ as well.

This illustration of one of our models with black hole spin = 0.10, adiabatic index = 1.4 , angular momentum =3.6[M] and specific energy= 0.0001 shows oscillation, shrinking and rebuilding of shock bubble (click on the image to see the animation). Those vertical oscillations of the shock bubble can be ascribed to the mixing of low and high angular momentum gas at the boundary of the funnel. This causes faster oscillations of the accretion rate with quite small amplitude and small motion of the shock front in the equatorial plane.
The shock shrinks towards the black hole when the gas flows through the outer part of the shock bubble and crosses the equator changing the physical conditions around the shock, providing a lot of gas with a much lower angular momentum. This increases the accretion rate and as soon this gas is accreted from the equatorial region, the shock bubble is rebuilt. (Simulation with the code HARM. Author: Ishika Palit)


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