New Benchmark of X-ray Line Emission Models of Fe XVII

Abstract

We review the accuracy of existing Fe XVII X-ray line emission models by comparing them with an extensive analysis of Chandra high energy transmission grating (HETG) observations of stellar coronae. We find significant discrepancies between most theoretical predictions and observations for at least some of the intensity ratios involving the six principal Fe XVII lines, 3C (15.01 Å), 3D (15.26 Å), 3E (15.45 Å), 3F (16.78 Å), 3G (17.05 Å), and M2 (17.10 Å). We suggest that the main problem of most previous theoretical studies to their inability to fully include electron correlation effects in the atomic structure calculations, while any deficiencies in the scattering approximation methods are of minor importance, regardless of it being close-coupling (CC) or distorted-wave (DW). An approximate method based on the many-body perturbation theory and DW approximation is proposed to include such correlation effects in the calculation of collisional excitation cross sections. The results are shown to agree with coronal observations and laboratory measurements better than most previous theories. Using the new atomic data, we then investigate the electron density sensitivity of the M2/3G intensity ratio and provide an improved density diagnostic tool for astrophysical observations.

High Resolution Photoexcitation Measurements Exacerbate the Long-Standing Fe XVII Oscillator Strength Problem

“…Our present work on possibly the most intensively studied many-electron ion in experiment and theory finally demonstrates the convergence of the atomic calculations on all possible parameters, removing incomplete inclusion of the correlation effects as a potential explanation of this puzzle. Shortcomings of low-precision atomic theory for L-shell ions had persistently been suggested based on the analysis of high-resolution Chandra and XMM-Newton data [13,18,19,23,58]. Similar inconsistencies were recently found in the high-resolution K-shell X-ray spectra of the Perseus cluster recorded with the Hitomi microcalorimeter [59,60].…”

“…A direct probe of these lines using an EBIT at the Linac Coherent Light Source (LCLS) X-ray free-electron laser (XFEL) found again their oscillator-strength ratio f (3C)/f (3D) to be lower than predicted, but close to astrophysical observations [24]. Difficulties with the calculations of oscillator strengths in many-electron systems [23][24][25][26][27][28][29][30] were highlighted. Due to the high peak brilliance of the LCLS XFEL, non-linear excitation dynamics [31,32] or non-equilibrium time evolution [33] might have affected that experiment.…”

“…However, for the past four decades, their observed intensity ratios persistently disagree with advanced plasma models, diminishing the utility of high-resolution X-ray observations. Several laboratory experiments using electron beam ion trap (EBIT) and tokamak devices scrutinizing plausible astrophysical and plasma physics explanations as well as the underlying atomic theory [15][16][17][18][19][20][21][22] have shown clear departures from predictions but confirmed astrophysical observations [18,19,23]. This has fueled a long-lasting controversy on the cause being a lack of understanding of astrophysical plasmas, or inaccurate atomic data.…”

X-ray spectra of the Fe-L complex II: atomic data constraints from EBIT experiment and X-ray grating observations of Capella

“…It is challenging to determine the resonant and continuum components simultaneously through fit, as their variations on the beam energy are both substantial. To overcome the degeneracy, we first fix the continuum spectrum to the theoretical value with the many-body perturbation theory (MBPT) method in the FAC code (Gu et al 2006b;Gu 2009). The calculation has been described in S19 and benchmarked within the same experiment.…”

“…Similar discrepancies are reported in Loch et al (2006); Chen (2011) with R-matrix approaches. As discussed in Gu (2009), Santana et al (2015), and S19, the issue is likely rooted on the theoretical treatment of the high-order electron correlation, which mainly affects the transition probability and cross section of the 2p 5 3d ( 1 P 1 ) level, and thus the 3C line emission at 15.01 Å. The other component in the 3d manifold, the 2p 5 3d ( 3 D 1 , so-called 3D) line seems to be better calculated (see Brown et al (2006)).…”