  We improve by a factor of 4-20 the energy accuracy of the strongest soft
X-ray transitions of Fe XVII ions by resonantly exciting them in an electron
beam ion trap with a monochromatic beam at the P04 beamline of the PETRA III
synchrotron facility. By simultaneously tracking instantaneous photon-energy
fluctuations with a high-resolution photoelectron spectrometer, we minimize
systematic uncertainties down to 10-15 meV, or velocity equivalent $\pm\sim$5
km s$^{-1}$ in their rest energies, substantially improving our knowledge of
this key astrophysical ion. Our large-scale configuration-interaction
computations include more than four million relativistic configurations and
agree with the experiment at a level without precedent for a 10-electron
system. Thereby, theoretical uncertainties for interelectronic correlations
become far smaller than those of quantum electrodynamics (QED) corrections. The
present QED benchmark strengthens our trust in future calculations of many
other complex atomic ions of interest to astrophysics, plasma physics, and for
the development of optical clocks with highly charged ions.
