Using the Livermore X-ray Spectral Synthesizer , which calculates spectral models of highly charged ions based primarily on HULLAC atomic data , we investigate the temperature , density , and photoexcitation dependence of the I ( 11.92 ~ { } { \AA } ) / I ( 11.77 ~ { } { \AA } ) line ratio of Fe xxii .
We find that this line ratio has a critical density n _ { c } \approx 5 \times 10 ^ { 13 } ~ { } cm ^ { -3 } , is approximately 0.3 at low densities and 1.5 at high densities , and is very insensitive to temperature and photoexcitation , so is a useful density diagnostic for sources like magnetic cataclysmic variables in which the plasma densities are high and the efficacy of the He-like ion density diagnostic is compromised by the presence of a bright ultraviolet continuum .
Applying this diagnostic to the Chandra High Energy Transmission Grating spectrum of the intermediate polar EX Hya , we find that the electron density of its T _ { e } \approx 12 MK plasma is n _ { e } = 1.0 ^ { +2.0 } _ { -0.5 } \times 10 ^ { 14 } ~ { } cm ^ { -3 } , orders of magnitude greater than that typically observed in the Sun or other late-type stars .