Rethinking Black Hole Accretion Discs
Author | |
Abstract |
<p>Accretion discs are staples of astrophysics. Tapping into the gravitational potential energy of the\ <span style="line-height: 1.6em;">accreting material, these discs are highly efficient machines that produce copious radiation and extreme\ </span><span style="line-height: 1.6em;">outfl</span><span style="line-height: 1.6em;">ows. While interesting in their own right, accretion discs also act as tools to study black holes and\ </span><span style="line-height: 1.6em;">directly \ </span><span style="line-height: 1.6em;">influence the properties of the Universe. Black hole X-ray binaries are fantastic natural laboratories\ </span><span style="line-height: 1.6em;">for studying accretion disc physics and black hole phenomena. Among many of the curious behaviors\ </span><span style="line-height: 1.6em;">exhibited by these systems are black hole state transitions\textendash\textendashcomplicated cycles of dramatic brightening\ </span><span style="line-height: 1.6em;">and dimming. Using X-ray observations with high temporal cadence, we show that the evolution of the\ </span><span style="line-height: 1.6em;">accretion disc spectrum during black hole state transitions can be described by a variable disc atmospheric\ </span><span style="line-height: 1.6em;">structure without invoking a radially truncated disc geometry. The accretion disc spectrum can be a powerful\ </span><span style="line-height: 1.6em;">diagnostic for measuring black hole spin if the effects of the disc atmosphere on the emergent spectrum are\ </span><span style="line-height: 1.6em;">well-understood; however, properties of the disc atmosphere are largely unconstrained. Using statistical\ </span><span style="line-height: 1.6em;">methods, we decompose this black hole spin measurement technique and show that modest uncertainties\ </span><span style="line-height: 1.6em;">regarding the disc atmosphere can lead to erroneous spin measurements. The vertical structure of the\ </span><span style="line-height: 1.6em;">disc is difficult to constrain due to our ignorance of the contribution to hydrostatic balance by magnetic\ </span><span style="line-height: 1.6em;">fields, which are fundamental to the accretion process. Observations of black hole X-ray binaries and the\ </span><span style="line-height: 1.6em;">accretion environments near supermassive black holes provide mounting evidence for strong magnetization.\ </span><span style="line-height: 1.6em;">Performing numerical simulations of accretion discs in the shearing box approximation, we impose a net\ </span><span style="line-height: 1.6em;">vertical magnetic \ </span><span style="line-height: 1.6em;">flux that allows us to effectively control the level of disc magnetization. We study how\ </span><span style="line-height: 1.6em;">dynamo activity and the properties of turbulence driven by the magnetorotational instability depend on the\ </span><span style="line-height: 1.6em;">magnetized state of the gas, spanning weak-to-strong disc magnetization regimes. We also demonstrate that\ </span><span style="line-height: 1.6em;">a background poloidal magnetic </span><span style="line-height: 1.6em;">flux is required to form and sustain a strongly magnetized accretion disc.\ </span><span style="line-height: 1.6em;">This thesis motivates the need for understanding how magnetic fields affect the observed spectrum from\ </span><span style="line-height: 1.6em;">black hole accretion discs.</span></p>
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Year of Publication |
2016
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Degree |
Ph.D.
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Number of Pages |
218
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Date Published |
07-2016
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University |
University of Colorado Boulder
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City |
Boulder, CO
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JILA PI Advisors | |
salvesenthesis.pdf21.82 MB
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Publication Status |