Oscillator-strength ratio of two Fe XVII soft X-ray transitions
  essential for plasma diagnostics finally agrees with theory
  One of the most enduring and intensively studied problems of X-ray astronomy
is the disagreement of state-of-the art theory and observations for the
intensity ratio of two ubiquitous Fe{~\sc XVII} transitions of crucial value
for plasma diagnostics, dubbed 3C and 3D. We unravel this conundrum at the
PETRA~III synchrotron facility by increasing the resolving power two and a half
times and the signal-to-noise ratio thousand-fold compared to previous work.
Hitherto unnoted Lorentzian wings of 3C, the stronger line, had been
indistinguishable from the background. The now-determined experimental
oscillator-strength ratio $R=
f_{\mathrm{3C}}/f_{\mathrm{3D}}=3.51\pm0.02_{\mathrm{stat}}\pm0.07_{\mathrm{sys}}$
agrees with our present prediction of $R=3.55\pm0.02$, calculated at the
largest scale reported for an astrophysical ion. Measured and predicted natural
linewidths also mutually agree. Further improvements would allow a test of key
QED contributions in this many-electron system.
