Summer REU Project: A 3D Analysis on Instabilities Disrupting Magnetically Arrested Disks
Northwestern 2020 CIERA Mentors: Nick Kaaz, Sophia Sanchez Maes, Dr. Alexander Tchekhovkskoy
About Me
Hello all, I am currently an undergraduate student at the University of San Diego in San Diego, California. I am in the class of 2023, and I'm currently pursuing a B.S. in Physics and Mathematics.
I'm interested in the intersections of astrophysics and tech, where I can help contribute to the growth of space exploration and aeronautics. A natural goal I'm working toawrd is expanding diversity in all areas of science and assisting the growth of black students in STEM fields.
You can contact me at koawotwi [at] gmail -dot- com.
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What is Black Hole Accretion?
Black hole (BH) accretion is a process where black holes are feed on accretion disks. An accretion disk is an orbital structure composed of gas and debris, and when gas accretes, it drags in another component associated with the disk: the magnetic field. The gas will drag the magnetic field into the event horizon, and this creates a tug of war between the intense gravitational forces and the magnetic pressure as a result of the field that was pulled into the black hole. At first, the black hole’s gravitational forces are dominant. After enough time has passed, the magnetic field will accumulate to a point of no return. The magnetic field will dominate the black hole’s gravitational forces, and the field that was dragged into the hole will be pushed out, forcefully.
Magnetically Arrested State (MAD state)
The MAD state occurs from when the pillow forms to after it disappears, and normal accretion occurs again. The MAD state is important because when the magnetic field is dragged into the BH, the magnetic flux increases as the magnetic pressure on the BH increases. At the highest flux, the field will be pushed out. This time can also represent the release of the strongest relativistic jets. The release of these jets reflect just how powerful this tug of war between the BH and the disk really iss. In 2D after the magnetic field is pushed out, this forms a sort of “magnetic pillow”. This pillow can be seen below in the 2D(top) vs. 3D(bottom) plot analyasis during the MAD state. Here the magnetic field lines are represented as the black lines. As you can see, the lines shield the gas from the black hole located at 0,0. This pillow impedes normal accretion, similar to 3D. For the sake of this project, I refer to the MAD state in 3D as imperfect only because the magnetic pillow that forms does not effectively halt accretion. The next section, Progression of MAD in 2D & 3D effectively shows the evolution of the MAD state, including the pillow phenomenon. The magnetic field looks different than in 2D, and that is for good reason. We found that in 3D, the gas accretes regardless of the pillow, and it resumes normal accretion much quicker than in two dimensions.
Progresion of MAD in 2D & 3D:
t = 8800
t = 9100
t = 9400
t = 9700
Emulating Accretion in Nature
Real Black hole accretion has to deal with the chaotic forces betwe in the tug of war I mentioned before, between the black hole’s gravitional forces and the magnetic field lines associated with the disk. This allows for exceptions that normally would not be witnessed in our 2D plots. These, are the instabilities that alter and disrupt the MAD state & the magnetic pillow. The instabilities are instances where gas accretes through local interactions of the tug of war (in clumps and pieces). We perturbed the simulation by adjusting the internal energy of the disk with an m=1 mode. The Black hole remains at (0,0), this time in the center. The disk, whose usual shape is a torus, is now axisymmetric due to the instabilities. This is a reflectin of the instabilities generated by the accretion disk's perturbed internal energy.
Next Steps?
Our 3D plots, which were made to replicate Black Hole Accretion in real conditions, show that realistic conditions break down the MAD state, as emulated by the instabilities in my project.
If we were to observe a black hole in nature, it’s going to vary extremely, so by understanding how unstable the magnetic pillow is we can set observationally how long the pillow and the MAD state lasts. That would be, in essence, how long it’s allowed to exist in nature
This is just one, of many steps we can take to understanding the Instabilities disrupting Magnetically Arrested Disks. From here, we can analyze different modes, like m=2, m=3. There are limits to my simulation. In nature, a disk accreting onto a Black hole will have more instabilities, or noise. This means that the types of perturbations during the MAD state, or the tug of war, will come in different forms and different intensities. But by investigating different modes with random chance, we can analyze the effects to apply it to nature. Eventually, we will be able to understand every way the MAD state breaks down.
