RUI: Understanding Black Hole Accretion Across a Range of Luminosities

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Stellar-mass black holes, meaning those only a few times more massive than the Sun, are detected via the light given off by matter falling into the black hole, a process that astronomers call accretion. This infalling matter is heated to high temperatures, causing it to radiate. Black hole accretion is a very complicated process: different black holes can have very different brightness, or luminosity, and the luminosity of an individual black hole usually varies considerably with time. A research group at the College of Charleston will study black hole accretion using numerical computer simulations. They will study three different accretion regimes, in distinctly different luminosity ranges, using cutting-edge simulations, and working with collaborators to compare their results directly with observations of the corresponding luminosity states. The goal is to solve some long-standing puzzles and attach physical explanations to observational phenomena. The expectation is that at least six undergraduate students will be involved in the project, gaining an exceptional opportunity to be exposed to forefront research in computational astrophysics. Additionally, the principal investigator will continue to disseminate his research results in a number of diverse forums, including engagements with the local arts community, presentations in local schools, YouTube videos, educational iPad apps, and Astronomy on Tap presentations.Tremendous progress has been made in recent years in understanding black hole accretion through numerical simulations. The addition of radiative cooling and radiation transport to such simulations over the past decade has made it possible to begin to make tighter connections and comparisons with observations. The research team will conduct three main sets of simulations, each targeting a specific luminosity range. Beginning at moderate luminosities, they will leverage the recent addition of multi-group radiation transport to their general relativistic radiation magnetohydrodynamics code to directly model the intermediate spectral states of X-ray binaries, including both the soft and hard photon contributions. At slightly higher luminosities, 10-20% of the Eddington limit, they plan to tackle the long-standing puzzle as to why disks in this range appear so stable in nature. Working from the hypothesis that strong magnetic fields may stabilize such disks, they plan to test multiple magnetic field configurations with the expectation that some may lead to high magnetization and stability. Lastly, they will extend their simulations beyond the Eddington limit to consider ultra-luminous X-ray sources (ULXs). The transformative aspect will be that the simulations will tilt, or incline, the accretion disk out of the black hole symmetry plane.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.