Theoretical, Experimental, and Observational Studies of Iron X-ray Spectra: From the Laboratory to the Universe

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2024

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The spectral lines of iron ions, particularly the dominant neon-like Fe XVII charge state, provide crucial diagnostics for the physical conditions of hot astrophysical plasmas in the X-ray regime. However, the diagnostic utility of these lines are hampered by significant discrepancies at the ~20% level between spectral observations, laboratory experiments, and theoretical calculations of the astrophysically important Fe XVII transitions, an issue that has been observed in numerous studies over several decades. Understanding the source of these discrepancies is critical for the improvement of both theoretical atomic models and laboratory experiment data on transition energies and cross sections of electron-ion processes, which themselves will be key for comparison to observations from X-ray spectroscopy missions such as XRISM, Line Emission Mapper (LEM), Arcus, and Athena. My dissertation encapsulates the main branches of X-ray astrophysics by focusing on the use of theoretical models and experimental measurements to further the diagnostic use, understanding, and interpretation of spectroscopic observations of iron transition lines.

I modeled the effects of UV photoexcitation in O-type stars on a spectral line ratio of the Fe XVII 3s – 2p transitions in an attempt to explain an anomalous value found for the X-ray spectra of the O star ζ Puppis. I conjectured that the strong UV field of ζ Pup produces the observed ratio by depopulation of metastable 3s excited states, and that the ratio can potentially be used as an independent diagnostic of the radial distribution of X-ray-emitting plasma. Using the Flexible Atomic Code (FAC) collisional-radiative model to model the effect of UV photoexcitation on the Fe XVII lines, I compared the model calculations to archival spectra of coronal and hot stars from the Chandra HETGS and XMM-Newton RGS. The calculations showed that UV photoexcitation does not produce a sufficiently large dynamic range in the Fe XVII line ratio to explain the difference in the observed ratio between coronal stars and ζ Pup.

I used FAC to compute steady-state populations of Fe XVII states and calculate cross sections for the dielectronic recombination (DR) and direct electron-impact excitation (DE) line formation channels of Fe XVII, and benchmarked the model predictions with experimental cross sections of Fe XVII resonances that were mono-energetically excited in an electron beam ion trap (EBIT) experiment. I extended the benchmark to all resolved DR and DE channels in the experimental dataset with a focus on the n ≥ 4 DR resonances, finding that the DR and DE absolute cross section predictions for the higher n complexes disagree considerably with experimental results when using the same methods as in previous works. However, agreement within ∼10% of the experimental results was achieved by an approach whereby I doubly convolve the predicted cross sections with both the spread of the electron-beam energy and the photon-energy resolution of the EBIT experiment. I also calculated rate coefficients from the experimental and theoretical cross sections, finding general agreement within 2σ with the rates found in the OPEN-ADAS atomic database.

Circling back to the ζ Pup Fe XVII ratio, I probed the potential significance of the process of resonant Auger destruction (RAD), which occurs when a photon emitted by an ion is absorbed in a neighboring cooler part of the stellar wind by near-coincident inner-shell transitions of lower charge state ions. The inner-shell excited ion then undergoes Auger decay, in which the energy is transferred to an outer electron that is subsequently ejected from the atom by autoionization. EBIT measurements at a synchrotron beamline determined that 3d – 2p transitions of the lower iron charge state Fe VI is nearly coincident in transition energy with the Fe XVII 3G line, which would enable possible destruction of Fe XVII 3G photons and thus a potential explanation of the lower line intensity ratio found in ζ Pup. Model calculations show a noticeable amount of optical thickness for the Fe VI line, but the calculated X-ray line profile model does not show nearly enough reduction of the Fe XVII 3G line to suggest that RAD by Fe VI lines is causing the ratio anomaly in ζ Pup.

Finally, I introduce preliminary steps for the analysis of XRISM spectral observations of Fe Kα lines from the starburst galaxy Messier 82. The key unsolved questions regarding M82 are what drives the hot wind and how much gas escapes the galaxy. Understanding the hot wind requires accurate measurements of its energy content, which requires obtaining constraints for the density, temperature, and velocity at the wind’s base. In order to sufficiently constrain the hot component velocity, the 6.7 keV Fe XXV line width and center must be determined to better than 10%. This accuracy requires an energy resolution ΔE ≤ 5 eV, which can be achieved by the high-resolution X-ray measurements with the XRISM Resolve calorimeter array. The M82 observation and subsequent analysis will confirm whether hot gas pressure is the primary driver of the galactic wind by measuring the energy contained in the T ∼ 10^8 K hot gas, and will constrain the mass-loading rate by measuring the velocity of the superheated nuclear gas using the Fe XXV line width.

By completing these works, I will have successfully contributed to the refinement and advancement of theoretical, laboratory, and observational X-ray astrophysical data for iron transition lines.

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