Using the Livermore X-ray Spectral Synthesizer , which calculates spectral models of highly charged ions based 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 .
Applied to the Chandra HETG spectrum of the intermediate polar EX Hya , we find that the electron density of its T _ { e } \approx 12 MK plasma is \log n _ { e } ( { cm ^ { -3 } } ) = 14.3 ^ { +0.7 } _ { -0.5 } , orders of magnitude greater than that observed in the Sun or other late-type stars .