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 Th⁶²⁺ and U⁶⁴⁺ show an intense line at 2558.7±{}0.2 and 2689.3±{}0.2 eV, respectively, that is attributed to the transition (3d⁹4s)J=3{→}(3d¹⁰{)}J=0$. Calculations indicate that the line is almost exclusively excited by indirect processes such as radiative cascades and electron capture.