Relativistic MHD Simulations of Tilted Black- Hole Accretion Disks A Study in Computational...
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Relativistic MHD Simulations of Tilted Black-Hole Accretion Disks A Study in Computational Astrophysics Presentation by Chris Lindner Research led by Dr. Christopher Fragile
Relativistic MHD Simulations of Tilted Black- Hole Accretion Disks A Study in Computational Astrophysics Presentation by Chris Lindner Research led by
Relativistic MHD Simulations of Tilted Black-Hole Accretion Disks
A Study in Computational AstrophysicsPresentation by Chris LindnerResearch led by Dr. Christopher Fragile
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Hello, my name is Chris Lindner. Today I will be presenting research results from a study I did with Dr. Fragile over the summer involving computational models of black hole accretion disks using the Cosmos++ code.
Black Hole Accretion Disks
What are they?
How do they form?
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Accretion disks are collections of matter that mass around a black hole. Because of their great gravitational pull, black holes tend to draw in large amounts of matter, and many times will feed off another star, creating a large, rotating disk around the hole. In most stellar systems, objects circling the star would generally stay in orbit. However, the density of the disk and spin of the black hole drive MRI, or magneto-rotational instability. This instability causes the matter around the black hole to lose orbital velocity, and eventually causes the matter to plunge into the black hole, where it cannot escape.
The Simulation
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The Cosmos++ code is designed to simulate an accretion disk surrounding a kerr black hole. Models can be simulated in 2d or 3d, though most must be simulated in 3d to obtain relevant results. The simulation divides the area around the black hole into a logarithmic grid. Each cell contains information about the matter in it, such as mass density, velocity, and magnetic fields. As the simulation progresses, the code calculates the gravitational and relativistic effects on the matter around the hole, and how it changes and the black hole rotates.
Tilted Disk Case• What is it?
• What evidence is there?
• Why is it interesting?
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Many research teams are currently studying models of accretion disks. One aspect of our research that makes it unique is our study of tilted accretion disks. In these simulations, the torus of mass surrounding the black hole is tilted away from the black hole's plane of spin.There is strong evidence to show that this may be very common in nature. A tilted disk can occur if the formation of the black hole is assymetric. A tilted disk may also be observed if a merger occurs between multiple black holes or between the supplying mass. Both of these situations are highly plausible and much oberservational data suggests that tilted disks may even be more common than standard disks.Tilted disks prove to be an interesting addition to Black Hole simulations as they add a three-dimensional angular component to these sytems. Adding in tilt creates a force that drives the disk to normalize itself with the plane of the black hole's spin.
Tilted Disk: What we Found
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What we expected to find in the tilted disk case is that, as matter approached the black hole, it would align itself along the plane of the spin of the hole. This graph outlines the average tilt of matter as it gets closer to the black hole. Contrary to our original hypothesis, we discovered that the matter took an extreme upward tilt as it approached the black hole, as can be seen on the left of the graph here. This result was highly counterintuitive and extremely puzzling.
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As we investigated, we found that in the mass density plot, as matter approached the black hole it formed two streams that indeed had tilted themselves away from the midplane of the black hole. This can be observed in this psuedocolor mass density video.
ISCOIn Newtonian Physics, a
particle can complete an elliptical orbit at any radius from the Black Hole
However, in Relativity, once a particlecrosses the ISCO, it can no longer
To find the answer to our dilemma, we had to look back at our black hole physics. In Newtonian physics, there is no minimum stable orbital radius for a particle circling a black hole; instead, particles could potentially maintain a stable circular orbit no matter how close they were to the black hole. At extremely small radii, the great gravitational force of the black hole would make the particle accellerate to the speed of light and beyond.However, in Relativity we find that this is simply not possible. Since matter cannot travel faster than the speed of light, we find that there must be a minimum boundary at which, for radii smaller than this boundary, no particle can maintain a stable circular orbit. This boundary is known as the Innermost Stable Circular Orbit, or ISCO.
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After computing the ISCO for each simulation, we found that these streams of matter appeared to form around the edges of the ISCO. It is here, beyond the ISCO, that matter in the disk loses stability. The angular momentum created by the disk's tilt causes matter to make one final partial rotation around the black hole before plunging in to the oblivion.These images show the mass density of the accretion disk surrounding the black hole. The ISCO is superimposed over each plot, illustrating its effect on the plunging streams near the black hole.
Future Studies•High resolution simulations•Effects of black hole parameters•Jets•QPO’s - Quasi-Period Oscillators•Photon and thermal Emissions
![Ken B. Henisey arXiv:1211.2273v2 [astro-ph.HE] 14 Nov 2012Variability from Nonaxisymmetric Fluctuations Interacting with Standing Shocks in Tilted Black Hole Accretion Disks Ken B](https://img.dokumen.tips/doc/110x75/60aeeb2336d56c21f9556ad0/ken-b-henisey-arxiv12112273v2-astro-phhe-14-nov-2012-variability-from-nonaxisymmetric.jpg)
