Observation of magnetic octupole decay in atomic spectra

We report the first observation of magnetic octupole decay in the x-ray spectrum of a highly charged ion. Detailed analyses of the low-energy region of the n=3→4 spectra of nickel-like ${\mathrm{Th}}^{62+}$ and ${\mathrm{U}}^{64+}$ show an intense line at 2558.7±0.2 and 2689.3±0.2 eV, respectively, that is attributed to the transition (3${\mathit{d}}^9$4s${)}_{\mathit{J}=3\mathrm{}}$→(3${\mathit{d}}^{10}$ ${)}_{\mathit{J}=0\mathrm{}}$. Calculations indicate that the line is almost exclusively excited by indirect processes such as radiative cascades and electron capture.