J. Drew Hogg

X-ray follow-up of two unusually red AGNs from the Swift Burst Alert Telescope (BAT) All-Sky Survey. We show that the AGNs are heavily obscured and possible analogs of extremely red objects (EROs), an elusive class of objects typically found at higher redshift.

Timing studies find that the variability from accreting black holes displays organized, nonlinear structure on viscous timescales. This is interpreted as evidence of the multiplicative combination of stochastic fluctuations in the mass accretion rate, though the model is largely phenomenological. Using a numerical model of an MHD accretion disk, I showed for the first time that these characteristic signatures naturally develop from MHD turbulence and that the magnetic dynamo drives their development.

Accretion disks with an inner truncation are invoked to explain the spectral properties of certain classes of black hole systems. The truncation is attributed to a transition in the radiative efficiency of the gas. Here, I study a hydrodynamic accretion disk with a bistable cooling law, meant to emulate such a transition. Several interesting features develop, which are distinct from the canonical model used to explain observations. This work establishes a benchmark against which an MHD model can be compared.

Here I continue the investigation into the behavior of a truncated accretion disk, but modeled in MHD. The dynamics, angular momentum transport, and energetics of the accretion flow are studied and reveal a gentle outflow originates from a "truncation zone." This leads to a subtle, but important, interaction between the hot gas and the cold Keplerian disk.

Throughout my accretion theory work, the role of the magnetic dynamo has been forefront in influencing the global disk evolution. To better understand the magnetic dyanmo and the large-scale magnetic field behavior, I also explored the dependence of the dynamo cycle on accretion disk thickness. The key finding is that the dynamo pattern fails to organize in thick disks.