Absolute measurement of the relativistic magnetic dipole transition in He-like sulfur

We have made an absolute, reference-free measurement of the $1s2s\phantom{\rule{0.28em}{0ex}}^{3}S_{1}\ensuremath{\rightarrow}1{s}^{2}\phantom{\rule{0.28em}{0ex}}^{1}S_{0}$ relativistic magnetic dipole transition in He-like sulfur. The highly charged sulfur ions were provided by electron-cyclotron resonance ion source, and x rays analyzed with a high-precision double-crystal spectrometer. A energy 2430.3685(97) eV was obtained is compared to most advanced bound-state quantum electrodynamics calculations, providing important test two-electron QED effects precision atomic structure methods medium-$Z$ species. Thanks extremely narrow natural linewidth this large dispersion spectrometer at energy, complementary study also performed evaluate impact different silicon crystal form-factor models transition-energy analysis. find no significant dependence on model used determine energy.