Polarization measurements of the Lyman-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>α</mml:mi><mml:mn>1</mml:mn></mml:msub></mml:math>x-ray emission lines of hydrogenlike<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi mathvariant="normal">Ar</mml:mi><mml:mrow><mml:mn>17</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi mathvariant="normal">Fe</mml:mi><mml:mrow><mml:mn>25</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math>at high electron-impact energies

We have measured the polarization of the $2{p}_{3∕2}\ensuremath{\rightarrow}1{s}_{1∕2}$ Lyman-${\ensuremath{\alpha}}_{1}$ x-ray line of hydrogenlike ${\mathrm{Ar}}^{17+}$ and ${\mathrm{Fe}}^{25+}$ at electron-impact energies ranging from 7 to 25 threshold units. The highly charged argon and iron ions were produced using the Lawrence Livermore National Laboratory SuperEBIT electron beam ion trap. A combination of two crystal spectrometers and a microcalorimeter were used to record the Lyman-$\ensuremath{\alpha}$ x-ray emission of ${\mathrm{Ar}}^{17+}$ and ${\mathrm{Fe}}^{25+}$ and to infer the polarization of the Lyman-${\ensuremath{\alpha}}_{1}$ line. Our results show a systematic discrepancy with the predictions of distorted-wave calculations.