Testing Astrophysical Models of Black Hole Accretion Disk Plasmas Using the Z machine at Sandia National Laboratories
Patricia Cho, University of Texas at Austin
The Z machine at Sandia National Laboratories generates powerful X-ray radiation fluxes. This enables experiments to produce and study macroscopic quantities of matter at extreme conditions. Z experiments expand our understanding of the essential physics for stockpile stewardship, nuclear fusion, and astrophysics. Astronomers use extraordinary spectra collected with satellite telescopes to construct models for the behavior of accretion powered plasmas around black holes in both active galactic nuclei and X-ray binaries. However, complex models for these radiation dominated non-Local-Thermodynamic-Equilibrium (NLTE) photoionized plasmas are mostly untested with laboratory data. A novel platform developed on the Z machine for expanding-foil photoionized plasma experiments opens a new regime for benchmark measurements of NLTE photoionized plasmas. The data from these experiments reveal difficulties in modeling both emission intensities and the level of ionization in the plasma. Such data have been a laboratory astrophysics goal for two decades but are even more critical now because of the “Super-Solar” iron abundance problem. Iron abundances in accretion disks inferred from X-ray spectra emitted by photoionized plasma in accretion disks that have formed around more than two dozen black holes appear to contain five to 20 times more iron than the Sun. This contradicts the widely held expectation that most objects in the universe have the Sun’s composition. One prevailing theory is that effects of high electron density are not properly accounted for in the models. Accretion disk plasma densities are now estimated to be orders of magnitude higher than previously believed. Reinterpreting the X-ray spectra with updated models resolved some of the discrepancy. However, a key question still remains: do spectral models of photoionized plasmas accurately account for X-ray emission? I will describe my progress in using this dataset to inform the Super-Solar iron abundance problem and discuss the broader potential to evaluate astrophysical model accuracy.