Rethinking Black Hole Accretion Discs

<p>Accretion discs are staples of astrophysics. Tapping into the gravitational potential energy of the\&nbsp;<span style="line-height: 1.6em;">accreting material, these discs are highly efficient machines that produce copious radiation and extreme\&nbsp;</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\&nbsp;</span><span style="line-height: 1.6em;">directly \&nbsp;</span><span style="line-height: 1.6em;">influence the properties of the Universe. Black hole X-ray binaries are fantastic natural laboratories\&nbsp;</span><span style="line-height: 1.6em;">for studying accretion disc physics and black hole phenomena. Among many of the curious behaviors\&nbsp;</span><span style="line-height: 1.6em;">exhibited by these systems are black hole state transitions\textendash\textendashcomplicated cycles of dramatic brightening\&nbsp;</span><span style="line-height: 1.6em;">and dimming. Using X-ray observations with high temporal cadence, we show that the evolution of the\&nbsp;</span><span style="line-height: 1.6em;">accretion disc spectrum during black hole state transitions can be described by a variable disc atmospheric\&nbsp;</span><span style="line-height: 1.6em;">structure without invoking a radially truncated disc geometry. The accretion disc spectrum can be a powerful\&nbsp;</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\&nbsp;</span><span style="line-height: 1.6em;">well-understood; however, properties of the disc atmosphere are largely unconstrained. Using statistical\&nbsp;</span><span style="line-height: 1.6em;">methods, we decompose this black hole spin measurement technique and show that modest uncertainties\&nbsp;</span><span style="line-height: 1.6em;">regarding the disc atmosphere can lead to erroneous spin measurements. The vertical structure of the\&nbsp;</span><span style="line-height: 1.6em;">disc is difficult to constrain due to our ignorance of the contribution to hydrostatic balance by magnetic\&nbsp;</span><span style="line-height: 1.6em;">fields, which are fundamental to the accretion process. Observations of black hole X-ray binaries and the\&nbsp;</span><span style="line-height: 1.6em;">accretion environments near supermassive black holes provide mounting evidence for strong magnetization.\&nbsp;</span><span style="line-height: 1.6em;">Performing numerical simulations of accretion discs in the shearing box approximation, we impose a net\&nbsp;</span><span style="line-height: 1.6em;">vertical magnetic \&nbsp;</span><span style="line-height: 1.6em;">flux that allows us to effectively control the level of disc magnetization. We study how\&nbsp;</span><span style="line-height: 1.6em;">dynamo activity and the properties of turbulence driven by the magnetorotational instability depend on the\&nbsp;</span><span style="line-height: 1.6em;">magnetized state of the gas, spanning weak-to-strong disc magnetization regimes. We also demonstrate that\&nbsp;</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.\&nbsp;</span><span style="line-height: 1.6em;">This thesis motivates the need for understanding how magnetic fields affect the observed spectrum from\&nbsp;</span><span style="line-height: 1.6em;">black hole accretion discs.</span></p>