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Laboratory tests of hypotheses for the super-solar iron abundance problem in black hole accretion disks

Presentation #111.04 in the session Laboratory Astrophysics Division (LAD): The Role of Laboratory Plasma Experiments in Astrophysics.

Published onJul 01, 2023
Laboratory tests of hypotheses for the super-solar iron abundance problem in black hole accretion disks

We will present data of the first ever high S/N iron L-shell x-ray emission spectra from a laboratory photoionized plasma. The data were obtained with the Z facility at Sandi National Laboratories. 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 surrounding about a dozen black holes appear to contain 5-20 times more iron than the Sun. This contradicts the widely held expectation that most objects in the universe have the Sun’s composition. Additionally, super-solar iron abundances are inferred for systems with vastly different parameters - stellar mass black holes in X-ray binaries as well as super massive black holes in AGN – likely indicating a systematic modeling error. One prevailing theory is that effects of high electron density are not properly accounted for in the models. Reinterpreting the x-ray spectra with updated models resolved much of that discrepancy partly because significant differences are predicted in the iron L-shell emission features. However, a key question still remains: do photoionized plasma spectral models accurately account for x-ray emission? We use a photoionized plasma platform developed over the last decade at Z to interrogate that question. We use the platform to create plasmas that reach the photon flux, density, and temperature conditions in a black hole accretion disk by controlling the composition, uniformity, and spatial expansion. We also measured photoionized calcium K-shell spectra as surrogates for iron K-shell spectra from black holes. The relativistically broadened iron K emission features also have the potential to strongly influence the inferred iron abundance. We will describe our progress in using this data to evaluate the model accuracy and its potential to inform the super-solar iron abundance problem.

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