Bridging the Gap Between Black Hole Accretion Disk Theory and X-Ray Spectral Observations

Accretion of material onto black holes and other compact objects is a fundamental process that shapes the growth and evolution of galaxies, star forming regions, and powers some of the most energetic phenomena in our universe (e.g. X-ray binaries, active galactic nuclei, gamma ray bursts, etc.). Black holes are notorious for their immense gravitational pull, stripping gas from stars which venture too close. Due to the angular momentum of the in-falling gas, a disk is formed around the black hole releasing incredible amounts of radiation, often at X-ray wavelengths. These accretion disks are extremely hot and luminous, which not only provide a way for us to detect and observe black holes, but also to study their only astrophysical properties: mass and spin. Modeling the complex accretion flows around black holes serves as one of the most direct ways we can obtain the spin of a black hole and test Einstein's theory of general relativity (GR) where the curvature of spacetime is at its most extreme. The foundational theory of accretion disks was developed almost 50 years ago, and it assumes that the gaseous material falling onto a black hole will take the form of a geometrically thin, optically thick accretion disk. The thin disk model is widely applicable to other areas of astronomy, such as proto-planetary disk systems and disks around accreting neutron stars and white dwarfs. It is commonly used as a first-order fit to the observed X-ray spectrum of accreting black holes. However, our theoretical understanding of black hole accretion has been challenged by observations of X-ray sources which are not well described by the thin disk model. An example of such sources are the well-known class of objects called ultraluminous X-ray sources (ULXs), which appear to accrete well above their Eddington limit, or the point at which the outward radiation pressure near the black hole exceeds the inward pull of gravity. Super-Eddington accretion onto black holes implies a much thicker accretion disk accompanied by radiatively driven outflows. This raises fundamental questions about the nature and physical mechanisms governing super-Eddington accretion, motivating the development of X-ray spectral models that can more accurately describe these flows. Due to the complex three-dimensional nature of super-Eddington accretion, numerical radiation magnetohydrodynamic (RMHD) simulations are required to replicate these environments.