M.F. Hasoglu

A Comprehensive X-Ray Absorption Model for Atomic Oxygen

An analytical formula is developed to accurately represent the photoabsorption cross section of O I for all energies of interest in X-ray spectral modeling. In the vicinity of the K edge, a Rydberg series expression is used to fit R-matrix results, including important orbital relaxation effects, that accurately predict the absorption oscillator strengths below threshold and merge consistently and continuously to the above-threshold cross section. Further, minor adjustments are made to the threshold energies in order to reliably align the atomic Rydberg resonances after consideration of both experimental and observed line positions. At energies far below or above the K-edge region, the formulation is based on both outer- and inner-shell direct photoionization, including significant shake-up and shake-off processes that result in photoionization-excitation and double-photoionization contributions to the total cross section. The ultimate purpose for developing a definitive model for oxygen absorption is to resolve standing discrepancies between the astronomically observed and laboratory-measured line positions, and between the inferred atomic and molecular oxygen abundances in the interstellar medium from XSTAR and SPEX spectral models.

K-shell Photoabsorption Studies of the Carbon Isonuclear Sequence

K-shell photoabsorption cross sections for the isonuclear C I-C IV ions have been computed using the R-matrix method. Above the K-shell threshold, the present results are in good agreement with the independent-particle results of Reilman & Manson. Below threshold, we also compute the strong 1s → np absorption resonances with the inclusion of important spectator Auger broadening effects. For the lowest 1s → 2p, 3p resonances, comparisons to available C II, C III, and C IV experimental results show good agreement in general for the resonance strengths and positions. Our results also provide detailed information on the C I K-shell photoabsorption cross section including the strong resonance features, since very limited laboratory experimental data exist. The resultant R-matrix cross sections are then used to model the Chandra X-ray absorption spectrum of the blazar Mkn 421.

Strong LSJ dependence of fluorescence yields: Breakdown of the configuration-average approximation

Using the five-electron K-shell vacancy 1s2s22p2 configuration as an example, we show that the fluorescence yields of the eight LSJ states of the configuration exhibit a dramatic dependence on LSJ. These results demonstrate that, in general, configuration-average fluorescence data are inappropriate for astrophysical modeling.

Importance of Configuration Interaction for Accurate Atomic Data: Fluorescence Yields of K-Shell Vacancy, Lithium-like Ions

We demonstrate that the inclusion of configuration interaction (CI) results in significant values for the K-shell fluorescence yields of Li-like ions, which are zero in a single-configuration approach. Modeling codes for simulating supernova remnants under nonequilibrium ionization conditions or photoionized plasmas such as active galactic nuclei or X-ray binaries need to be updated accordingly. A two-parameter fitting formula for the fluorescence yields has been developed. The generality of important CI effects on atomic calculations is pointed out.

K-shell photoabsorption of magnesium ions

X-ray photoabsorption cross sections have been computed for all magnesium ions with three or more electrons using the R-matrix method. A comparison with other available data for Mg II-Mg X shows good qualitative agreement in the resultant resonance shapes. However, for the lower ionization stages, and for singly ionized Mg II in particular, the previous R-matrix results overestimate the K-edge position due to the neglect of important orbital relaxation effects, and a global shift downward in photon energy of those cross sections is therefore warranted. We have found that the cross sections for Mg I and Mg II are further complicated by the M-shell (n = 3) occupancy. As a result, the treatment of spectator Auger decay of 1s → np resonances using a method based on multichannel quantum defect theory and an optical potential becomes problematic, making it necessary to implement an alternative, approximate treatment of Auger decay for neutral Mg I. The new cross sections are used to fit the Mg K edge in XMM-Newton spectra of the low-mass X-ray binary GS 1826-238, where most of the interstellar Mg is found to be in ionized form.

Photoionization of Xe@C60 using R-matrix Methods

We report Breit-Pauli and Dirac R-matrix cross sections for the photoionization of endofullerene Xe@C60, showing strong confinement resonances, in excellent agreement with a recent experiment.

K-shell photoionization of oxygen

R-matrix calculations of the 1s photoionization in oxygen are reported. Proper treatment of spectator Auger broadening, relaxation, pseudoresonace elimination, and shake-up/shake-off are addressed.

Radiation damping in the photoionization of Fe14

Pauli R-matrix formulation including both fine-structure splitting of strongly-bound resonances and radiation damping. The radiation damping of 2p ! nd resonances gives rise to a resonant photoionization cross section that is significantly lower than the total photoabsorption cross section. Furthermore, the radiation-damped photoionization cross section is found to be in good agreement with recent experimental results once a global shift in energy of � 3.5 eV is applied. These findings have important implications. Firstly, the presently available synchrotron experimental data are applicable only to photoionization processes and not to photoabsorption; the latter is required in opacity calculations. Secondly, our computed cross section, for which the L-shell ionization threshold is aligned with the NIST value, shows a series of 2p ! nd Rydberg resonances that are uniformly 3-4 eV higher in energy than the corresponding experimental profiles, indicating that the L-shell threshold energy values currently recommended by NIST are likely in error.

X-ray photoabsorption in magnesium ions near the K-edge

K-shell photoabsorption cross sections for magnesium ions are computed using an R-matrix method.