First application of the Fe XVII I(17.10 angstrom)/I(17.05 angstrom) line ratio to constrain the plasma density of a cosmic X-ray source

We show that the Fe XVII I(17.10 Angstrom)/I(17.05 Angstrom) line ratio obs erved in the Chandra High-Energy Transmission Grating (HETG) spectrum of th e intermediate polar EX Hya is significantly smaller than that observed in the Sun or other late-type stars. Using the Livermore X-Ray Spectral Synthe sizer, which calculates spectral models of highly charged ions based on HUL LAC atomic data, we find that the observed I(17.10 Angstrom)/I(17.05 Angstr om) line ratio can be explained if the plasma density n(e) greater than or similar to 3 x 10(14) cm(-3). However, if photoexcitation is included in th e level-population kinetics, the line ratio can be explained for any densit y if the photoexcitation temperature T-bb greater than or similar to 55 kK. For photoexcitation to dominate the Fe XVII level-population kinetics, the relative size of the hot spot on the white dwarf surface must be f less th an or similar to 2%. This constraint, and the observed X-ray flux, requires a density n greater than or similar to 2 x 10(14) cm(-3) for the post-shoc k flow. Either way, then, the Chandra HETG spectrum of EX Hya requires a pl asma density that is orders of magnitude greater than that observed in the Sun or other late-type stars.