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

TL;DR

This study uses high-resolution photoexcitation measurements to address the longstanding discrepancy between observed and theoretical oscillator strength ratios of Fe XVII's key x-ray transitions, providing new experimental data that challenge recent calculations.

Contribution

The paper presents the first high-resolution synchrotron measurements of Fe XVII oscillator strengths, significantly reducing uncertainties and revealing a persistent discrepancy with recent ab initio theoretical predictions.

Findings

Measured oscillator strength ratio f(3C)/f(3D) = 3.09(8)(6)

Supports earlier astrophysical and laboratory observations

Challenges recent large-scale theoretical calculations

Abstract

For more than 40 years, most astrophysical observations and laboratory studies of two key soft x-ray diagnostic $2p-3d$ transitions, $3C$ and $3D$, in Fe XVII ions found oscillator strength ratios $f(3C)/f(3D)$ disagreeing with theory, but uncertainties had precluded definitive statements on this much studied conundrum. Here, we resonantly excite these lines using synchrotron radiation at PETRA III, and reach, at a millionfold lower photon intensities, a 10 times higher spectral resolution, and 3 times smaller uncertainty than earlier work. Our final result of $f(3C)/f(3D) = 3.09(8)(6)$ supports many of the earlier clean astrophysical and laboratory observations, while departing by five sigmas from our own newest large-scale ab initio calculations, and excluding all proposed explanations, including those invoking nonlinear effects and population transfers.